Introducing Physical Geography, 6th Edition Solution Manual

Type:

Solution Manual

Resource:

Introducing Physical Geography

Edition:

6th Edition

Author(s):

Alan Strahler

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 1: Earth as a Rotating Planet Chapter Objectives: Upon completion of this chapter the student will be able to: 1) Describe the shape of the Earth.

2) Explain Earth’s rotation.

3) Describe the features of the geographic grid.

4) Explain different types of map projections.

5) Describe Earth’s time zones.

6) Describe Earth’s seasons using its revolution around the Sun. This chapter introduces a few of the fundamental tools that geographers use including maps, GIS, and GPS. There are a number of ways that we can introduce these topics to appeal to the variety of interests and learning styles of your students.

A Historical Perspective: For many non‐science students, topics in physical geography can seem distant and unfamiliar. The historical aspects of maps, and the ways in which we have developed these techniques over hundreds of years, provide a useful method to instill interest and enthusiasm in these students. The following list of internet resources furnishes students with interactive learning opportunities, creatively explaining each of the major topics presented.

Helpful Hints: • Following a review of seasons and daylight, assign students to follow the animations in WileyPLUS Follow up in WileyPLUS with a quick quiz.

• Provide students with a list of world cities and, with the aid of an internet resource such as the WorldTime Interactive Atlas, have them calculate the different times in each of the cities when it is 9:00 am local time.

• Similarly, provide students with a list of dates and assign them to look at the Guide to Arctic Sunrise and Sunset site to find out how many hours of daylight they would receive.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface to their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are organized for viewing by core concept and by region.

GeoDiscoveries animations and interactivities illustrate and reinforce key concepts. They are good for use in lectures and as demonstrations. Students can use the activities for practice or study.

Nova Lost at Sea: The Search for Longitude. This web site, from the PBS series Nova, outlines the problems involved with developing an accurate system of longitude for navigation. http://www.pbs.org/wgbh/nova/longitude/ NOAA Digital Topographic Atlas provides many different topographic maps using digital elevation models. http://www.ngdc.noaa.gov/mgg/topo/topo.html This short webpage is a good introduction to Geographic Information Systems (GIS). http://www.gis.com/content/whatisgis WorldTime Interactive Atlas provides an interactive globe that allows you to point and click to find out the longitude, latitude and current time. http://www.worldtime.com/ World Time Server provides the current time and location of any country, including the different states of the US and provinces of Canada. http://www.worldtimeserver.com/ Guide to Arctic Sunrise and Sunset. This web site provides the hours of daylight and darkness for each day of the year for three latitudes in the Arctic region. http://www.athropolis.com/sun‐fr.htm SunAngle: Calculates solar angles and related information for a given location, date, and time. http://www.susdesign.com/sunangle/ SunPosition: Calculates the position of the sun and related information for a given location, date, and time. http://www.susdesign.com/sunposition/ Interactive World Time Zone Map provides a variety of ways to view the world’s different time zones. http://www.worldtimezone.com/

Tracking the North Magnetic Pole. It is important to remember that the geographic poles and the magnetic poles are different. This Canadian web site demonstrates how the magnetic north pole is moving. http://geomag.nrcan.gc.ca/mag_fld/magdec-eng.php Multimedia Tour of the Sun. This site from the Netherlands gives a clear and concise tour of the Sun and explains the processes and features of our nearest star. http://www.michielb.nl/sun/ Animation of the seasons. This web site shows Earth orbiting around the Sun and how the declination changes. It has a useful stop feature so you can pause to evaluate the seasonal changes. http://www.astro.uiuc.edu/projects/data/Seasons/seasons.html Tutorial on Global Positioning Systems (GPS) by Trimble, a leading manufacturer of GPS equipment. http://www.trimble.com/gps_tutorial/ Introduction to LiDAR (Light Detection and Ranging), a relatively new airborne mapping technique. http://www.csc.noaa.gov/digitalcoast/_/pdf/What_is_Lidar.pdf

Chapter Lecture 1.1 The Shape of Earth: The Earth is not a perfect sphere, as evidenced by the 26 mi (42 km) difference between the equatorial diameter of 7,926 mi (12,756 km) and the polar diameter of 7,900 mi (12,714 km). The reason for this difference, even if it is slight, is due to the centrifugal force exerted on the planet by the Earth’s rotation. Thus, the equatorial bulge and the polar flattening make the Earth’s shape an oblate ellipsoid rather than a sphere. Actually, an even better characterization of the Earth’s shape is the geoid, which reflects the unevenness of gravitation pull.

1.2 Earth’s Rotation: Earth rotates on an imaginary straight line through the poles and center.

One complete rotation is referred to a solar day that is divided into 24 hours. This axial rotation provides a ready reference for establishing a geographic grid of latitude and longitude, in addition to the convenience of having the day divided in hours, minutes, and seconds.

1.2.1 Environmental Effects of Earth’s Rotation: These include the following factors that are of substantive importance to all life forms: sunlight, air temperature, humidity, and motion are all affected by this diurnal rhythm. The Coriolis effect is a rightward deflection of air and ocean currents in the northern hemisphere that is caused by Earth’s rotation and varies from zero at the Equator to a maximum at the poles. The deflection is leftward in the southern hemisphere. Ocean tides on Earth are caused by the gravitational attraction of the moon and the sun as the planet rotates on its axis.

1.3 The Geographic Grid: An orderly system of imaginary circles, called meridians and parallels, is used to locate any position on Earth. It is necessary to establish these circles in order to make maps.

1.3.1 Parallels and Meridians: Parallels (of latitude) are east‐west circles that are parallel to one another. The equator at 0º is the largest parallel. We can define as many parallels as we want.

1.3.2 Meridians (of longitude) are north‐south lines that extend from one pole to another.

As with parallels, we can define as many as we want.

1.3.3 Great circles pass through the center of Earth.

1.3.4 Small circles pass through Earth but not through its center. Meridians are halves of great circles, while all parallels, with the exception of the equator, are small circles.

1.3.5 Latitude and longitude are used to pinpoint any location on Earth’s surface. Latitude is measured as so many degrees north or south of the equator to either pole, thus ranging from 0º to 90º. In comparable fashion, longitude is measured as so many degrees east or west of the prime meridian, the 0º reference point passing through Greenwich, England.

Prime Meridian Controversy Discussion: The class should be interested to know that there was disagreement among some of the European nations and the United States about where this special meridian should be established. Some European states preferred to use the meridian of Ferro, going through the westernmost part of the Canary Islands (off the southwest coast of Morocco), using the rationale that all of Europe would then lie east of the prime meridian. Indeed, the Ferro (or Hierro Island) meridian was adopted for French maps by King Louis XIII in 1634.

To make matters more complicated, approximately 18 countries wanted to have the prime meridian for the world based on their own local national meridians, usually referring to a particular site in their capital city. The list included Paris, Rome, Washington, DC, Istanbul, Turkey, and Jakarta, Indonesia. The matter was finally settled at the International Meridian Conference in Washington, DC in 1884 with an agreement that the prime meridian for the world would pass through the Royal Observatory at Greenwich near London, England. The meeting was attended by 41 delegates from 25 countries. The final vote was 22 in favor, 1 against (Dominican Republic), and 2 abstentions (France and Brazil). If you think that this conference was the final act, it took until 1909 for most nations to accept the Greenwich, England location.

And if you think that was the end of the story, Swiss railroad maps printed in 1916 still used Paris as the prime meridian. One could comment further on Anglo‐French discord, but enough said.

1.4 Map Projections: Cartography represents the field of mapmaking. The major problem for mapmakers over the centuries has been the difficulty of making usable flat maps of a spherical planet. Unless one is using a globe, all maps have some degree of distortion, ranging from minor to major. The goal then is to select a particular projection that best fits the needs of the user.

1.4.1 The Polar Projection is part of the zenithal or azimuthal class. It is centered around a point, such as the North (or South) Pole. This projection has a radial symmetry and can cover an area from the Equator to either pole. All the meridians are straight lines that radiate out from the center point or pole. The parallels of latitude are concentric circles that increase as they head to the periphery of the map. Accordingly, scale increases away from the center point or pole.

1.4.2 The Mercator Projection was developed in 1568 by Gerardus Mercator, a Flemish geographer, mathematician, and cartographer. It is a type of conformal projection in which lines of latitude and longitude cross each other at right angles. This featureis not true of all maps that have right‐angle crossings. Mercator maps have special properties, such as a line of constant compass bearing that appears as a straight line and can therefore be used to show directional features such as wind and ocean currents, volcanic chain movements, and lines of equal air temperature and pressure.

Mercator Discussion: Based on the “overuse” of the Mercator projection in routine use, the class should be strongly advised that, although it is one of the few conformal maps that can show large areas of the world, it is particularly inappropriate for world depiction. The distortion of size is very large, particularly as the map scale increases very rapidly in a poleward direction. Thus, at latitudes between 60º and 80º, the scale is two times to six times greater than at the Equator.

Another way to get the point across to the class is to simply show a Mercator map of the world and point out the enormous size of Greenland as compared to the entire continent of South America. Then write on the board that Greenland is actually 840,000 sq mi (2,175,600 sq km) as compared to 6,879,916 sq mi (17,818,928 sq km) for all of South America.

1.4.3 Conformal and Equal-Area Maps: In Conformal map projections, the land area represented changes, while the shapes remain constant. The Mercator projection is an example of a conformal map. In an Equal-Area projection, the land areas are correctly depicted.

1.4.4 The Winkler Tripel Projection shows the land areas of the world with limited distortion of shape and area. Although this type of projection is neither conformal nor equal‐area, it is the best suited for properly displaying global information, such as climate, soils, and vegetation.

1.5 Global Time: Planet Earth needs 24 hours to rotate fully with respect to the sun. Therefore, if it turns 360º/day, the rotation rate is 360º/24 = 15º/hour. Accordingly, 15º of longitude works out to be one hour of time.

1.5.1 Standard Time requires that Earth be divided into time zones that are one hour apart.

There are some places, such as Newfoundland, that prefer to be ½ hour apart for local convenience.

1.5.2 World Time Zones total 24. Russia, given its enormous east‐west spread, actually covers 11 times zones but groups them together into eight zones. Many countries change the extent of their time zones for convenience. For example, the Gary area in northwestern Indiana is placed in Central Time due to its proximity to Chicago. Another example is El Paso in southwestern Texas, which is placed in Mountain Time due to its distance from the rest of Texas.

Suggested Time Problems Discussion: Example #1 refers to local mean time, which is based on the local meridian of longitude. Draw two long vertical lines on the board. Label the one on the right as Portsmouth, New Hampshire at 70º 50' W. Long. and on the left as Manchester, New Hampshire at 71º 30' W. long. Tell the class that the east‐west difference between the two cities is 40 mi (64.4 km). If we were not using the current standard time system, the local time would be 2.7 minutes ahead in Portsmouth as compared to Manchester. Now locate (very approximately with a large “X”) a point on the southern part of the same vertical meridian that Manchester is on and label that “X” as a small village close to Valparaiso, Chile. The local mean time for both locations in Manchester, NH and the village in Chile is the same, but they are 5,200 mi (8,367 km) apart.

Example #2 provides another example of the difference between standard and local time. Draw three solid vertical lines and label them 60º, 75º, and 90º W. long. Between these solid lines, draw two dashed vertical lines and label them 67º and 86.5º W. long. Label the one on the right as Eastport, ME (67º W) and the one on the left as Benton Harbor, MI (86.5º W). Eastport is one of the easternmost locations in the U.S. and actually lies within the Atlantic Standard Time zone that includes the Maritime Provinces of Canada (New Brunswick, Prince Edward Island, and Nova Scotia). For obvious reasons of convenience, it is included in the Eastern Standard Time zone. In a similar manner, Benton Harbor on Lake Michigan is technically within in the Central Standard Time zone. The difference in local time between Eastport and Benton Harbor is 78 minutes, but both places are in the same time zone for convenience.

1.5.3 The International Date Line is set at the 180º meridian, which fortunately is located in the middle of the Pacific Ocean. This is a wonderful place to start the new day on Earth. This line is permitted to shift in longitude as a convenience for national boundaries.

Two examples are: 1) the bending of the Date Line between Alaska and Russia, and 2) the shift in the Date Line in the South Pacific to keep clear of New Zealand and some islands. Thus, if one is traveling eastward across the 180º meridian, the clocks are set back by one day. Conversely, if you are traveling westward, your clock would be advanced by one day.

International Date Line Enigma: Identify the International Date Line on a globe. Ask anyone in the class how long Monday or Tuesday, etc. exists on Earth. The most probable response would be one day. Be gentle, but tell them they are wrong. Now take the globe, spin it to the International Date Line, and tell them that they should assume that the 180º meridian has an imaginary slit in it. You now ask them to further assume that you will be drawing an imaginary roll of transparent plastic wrap from the slit and pulling it out very slowly. Your direction of movement will go from the International Date Line at the 180º meridian to 165º E. long. Assume the leading edge of the plastic wrap is the first minute of the new day, say Monday. It will take the new day of Monday one hour to reach 165º E. long., two hours to reach 150º E. long., 12 hours to reach the prime meridian at 0º, and 24 hours to return to the International Date Line at the 180º meridian. At this moment, the entire planet is covered by Monday. We now must get rid of Monday as the leading edge of Tuesday is starting to emerge from the slit. Let us assume that Monday starts to go back into the slit at the same rate as it came out. If it took Monday 24 hours to emerge from this slit, it will take 24 hours for Monday to descend back into the slit. You have now witnessed why Monday, and all other days, lasts 48 hours on the Earth. The class should feel free to try this out on their parents, relatives, and friends as a display of their newfound knowledge about the many aspects of physical geography.

1.5.4 Daylight Saving Time occurs during that part of the year when there is a longer period of daylight so that our work activities can take place when more people are awake and busy. It is a simple method of moving all clocks ahead by one hour. The current change for the U.S. occurs on the second Sunday in March and ends on the first Sunday in November. For a variety of reasons, Arizona, some counties in Indiana, and places like Hawaii, Puerto Rico, and other islands in low‐latitude locations remain on standard time.

Daylight Saving Time was adopted by the U.S. during World War I and could be used afterwards ifauthorized by local legislation; it was used in the U.S. again during World War II. By virtue of its higher latitudinal position, England decided to double its daylight saving time during World War II and, in the process, called it summer time.

1.5.5 Precise Timekeeping is now accomplished by a worldwide system of master atomic clocks that are able to measure time to an astonishing accuracy of more than one part in one trillion.

1.6. Earth’s Revolution Around the Sun: In addition to rotating on its axis, the Earth revolves about the sun on an imaginary horizontal surface called the plane of the ecliptic. A complete revolution around the Sun takes 365.242 days. In order to account for this fraction of a day, an extra day (February 29) is added to the calendar once every four years, producing a leap year.

Other characteristics of Earth’s orbit include the following: 1) The orbit of Earth around the Sun is slightly elliptical (or oval), 2) Earth is closest to the Sun on January 3, a position that is referred to as perihelion, from the Greek words peri- (around or near) and helios (the sun), 3) Earth is farthest from the Sun on July 4, a position that is referred to as aphelion, from the Greek words ap- (away from) and helios (the sun), and 4) Earth revolves counterclockwise around the Sun, a movement that is similar to the axial rotation of Earth and also of most of the other planets in the solar system.

How Far is Earth from the Sun? Exercise One: Try this out on your class, as it is straightforward but has a slight twist to it. Earth’s distance from the Sun at perihelion is 91.5 million mi (147 million km), whereas the distance at aphelion is 94.5 million miles (152 million km). The difference between these distances is approximately 3%. Now ask the class if they think that being closer to the Sun in January and FARTHER from the Sun in July should make the climate warmer and then colder in the northern hemisphere. If they remain uncertain, just hint that it will be discussed later on.

Exercise Two: Ask the class what would happen hypothetically if the thermonuclear reactions on the Sun that produce light and heat were suddenly to stop. You can expect all kinds of comments about the onset of nasty scenarios, but you may want to hint about Earth’s distance from the sun.

Let us take the average distance between perihelion and aphelion as about 94.5 million miles (149.5 million km) as a starter. Light travels at the speed of 186,000 mi/sec (299,274 km/sec).

Therefore, it takes about 8.5 minutes for the light from the sun to reach Earth. Let the class cogitate about what happens next.

1.6.1 Motions of the Moon include its counterclockwise axial rotation and revolution around Earth. The Moon’s rotation rate is synchronized with Earth’s rotation, so that we can only see a bit more than 50% of its surface. The far side of the moon was unknown until 1959, when a Russian spacecraft was able to take photos of it.

1.6.2 Tilt of Earth’s Axis refers to the 66.5º angle between the polar axis and the imaginary surface in space referred to as the plane of the ecliptic. Another way of looking at it is to indicate that Earth’s polar axis is inclined 23.5º away from a right angle to this plane of the ecliptic. This axial inclination points to a position in space close to Polaris, the North Star.

1.6.3 The Four Seasons on Earth occur because the northern hemisphere is tilted away from the Sun during one part of the year and tilted toward the Sun during another part.

This axial tilt that occurs as Earth revolves around the Sun gives rise to the different seasons, as follows. The Winter Solstice occurs on December 21 or 22, when the northern hemisphere is tilted away from the Sun and therefore receives the least amount of sunlight. The Summer Solstice occurs on June 21 or 22 when the northern hemisphere is tilted towards the Sun and therefore receives the greatest amount of sunlight. The Vernal Equinox and the Autumnal Equinox occur on March 20 or 21 and September 22 or 23,

respectively. At that time, neither pole is tilted toward the sun, thereby resulting in equal lengths of day and night.

Comment on Solstice and Equinox Dates: The aforementioned two calendar dates given for each of the solstices and equinoxes simply reflects the 365.25 days that it actually takes Earth to revolve about the Sun. As a result, leap years were established every four years, thereby necessitating the two dates.

1.6.4 Equinox Conditions are based on the fact that Earth is always divided into two hemispheres—one in sunlight and the other in darkness. The line on Earth that separates the day hemisphere from the night hemisphere is called the circle of illumination.

Another term, the subsolar point, refers to the single point on the Earth’s surface where the Sun is directly overhead at a particular time. On the date of the Equinox, the length of day and night is12 hours each.

1.6.5 Solstice Conditions reflect the fact that the Sun’s rays of light and heat are now spread on either the northern or southern hemisphere depending upon the season. The hours of day and night will now vary from zero to 24 hours. Locations in the midlatitudes, approximately 40º N. lat., will have 9 and 15 hours of daylight during the winter and summer solstices, respectively. Certain parallels of latitude are assigned different names, such as the Tropic of Cancer and Capricorn at 23.5º N and S, and the Arctic and Antarctic Circle at 66.5º N and S, respectively. The subsolar points range from 23.5º N to 23.5º S, based on the previous discussion indicating that Earth’s polar axis is inclined 23.5º away from a right angle to the plane of the ecliptic.

Suggestion: Explain to the class that the Sun can only be directly overhead at 90º at noon on those places on Earth that lie between the Tropics of Cancer and Capricorn. Now ask the class what the latitudinal location would be for the Tropics of Cancer and Capricorn if, hypothetically, the Earth’s axial inclination would change from 23.5º to 30º. This may be too easy, but the correct answer would be 30º N and S.

Select Audiovisual Aids: Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126). Understanding Time.

DVD OVU 8941‐KS; VHS OVU 8941‐A, 53 minutes, 1996. Insight Media, New York, NY (800‐233‐9910).

Understanding and Calculating World Time. VHS # 79‐AM‐36, 16 minutes, 2002. Maps and Globes: A Thorough Understanding. DVD # 79‐AS‐967, 20 minutes, 2004. Many Ways to See the World. DVD # 79‐AS‐973, 30 minutes, 2005.

Select Reference Books: Campbell, John. Map Use and Analysis. New York, NY: McGraw‐Hill, 4th ed., 2001.

Dent, Borden D., Jeffrey S. Torguson, and Thomas W. Hodler. Cartography: Thematic Map Design. New York, NY: McGraw‐Hill, 6th ed., 2009.

Dorling, Daniel and David Fairbairn. Mapping: Ways of Representing the World. Essex, England: Longman, 1997.

Krygier, John and Denis Wood. Making Maps: A Visual Guide for Map Design for GIS. New York, NY: Guilford Press, 2005.

Monmonier, Mark and George A. Schell. Map Appreciation. Englewood Cliffs, NJ: Prentice Hall, 1988.

Monmonier, Mark. Coastlines: How Mapmakers Frame the World and Chart Environmental Change. Chicago, IL: University of Chicago Press, 2008.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 2: The Earth’s Global Energy Balance Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe Earth’s absorption and emission of electromagnetic radiation.

2) Describe the variation of incoming solar radiation by latitude and season over the Earth.

3) Describe the composition of Earth’s atmosphere.

4) Compare sensible and latent heat.

5) Describe the flow of energy from the Sun to Earth’s atmosphere and surface and the return flow to space.

6) Explain how net radiation varies with latitude.

This chapter introduces a number of concepts that non‐science students, in particular, may find unfamiliar. In addition, many of the concepts must be thoroughly understood in order for subsequent topics to make sense.

The Electromagnetic Spectrum: The electromagnetic spectrum is often unfamiliar to students but is, of course, important for understanding a wide variety of topics. Assign your students to review the animated spectrum in WileyPLUS and then point them to some of the sites below to identify wavelengths used in satellite sensors.

The Atmosphere: As the atmosphere is described by its different layers, it can be all too tempting for students to learn about its various characteristics by simply memorizing the individual layers and their properties, rather than fully understanding their function and interaction.

Remote Sensing: One of the major areas of interest for geographers is in the rapidly expanding field of geographic technologies such as remote sensing. The sites provided here, such as Space Imaging Gallery, are useful in introducing such technologies either briefly in class or as an out-of-class assignment for students to find further information. For example, show your students the NASA Earth Observatory site and the many images of current events, such as forest fires, that it presents.

The Ozone Layer: Ozone depletion is a topic that most students have heard about, but many may be unclear as to the causes and effects. Sites from the EPA and Environment Canada are useful sites from which students can obtain more specific information.

The Energy Balance: An interactive exercise in WileyPLUS provides a basic introduction to energy-balance models and some of their simulations. For students who would like to try a model for themselves, two online examples are provided below with some guidance on their use. For example, challenge students to reproduce some of the results in the WileyPLUS interactivity.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface to their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are organized for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

NASA’s Earth Observatory: http://earthobservatory.nasa.gov TerraServer—fine-resolution satellite imagery. Take a look at this site with some of the most detailed satellite images available today: http://www.terraserver.com/ Remote Sensing Core Curriculum is a detailed web site describing and explaining the field of remote sensing: http://www.research.umbc.edu/~tbenja1/umbc7 An Energy Balance Climate Model developed by the Shodor Educational Foundation. Change the variables including the solar constant and ice and cloud albedos to produce a simple simulation. http://www.shodor.org/master/environmental/general/energy/index.html The official web site for CERES (Clouds and the Earth’s Radiant Energy System). http://ceres.larc.nasa.gov/ A Primer on Ozone Depletion (U.S. Environmental Protection Agency). A general introduction to and review of the problems and impacts associated with stratospheric ozone depletion. http://www.epa.gov/ozone/science/index.html

Chapter Lecture: 2.1 Electromagnetic Radiation: Electromagnetic radiation is a collection of waves with widely varying wavelengths that travels away from an object’s surface. Heat and light are both forms of electromagnetic radiation, but differ in wavelength—the distance between wave crests. The unit used to measure wavelength is the micrometer, equal to one millionth of a meter. The electromagnetic spectrum ranges over an incredible array of wavelengths of which visible light covers only one small segment. Gamma and X-rays have extremely short wavelengths that are associated with high energies. These high-energy wavelengths are extremely dangerous, due primarily to their ability to damage living tissue.

Suggestion: In case anyone in the class is unaware of health problems associated with the very-short-wavelength part of the spectrum, ask why lead shields are routinely used for dental X-rays.

2.1.1 Radiation and Temperature: There are two important physical principles that pertain to electromagnetic radiation emission. The first is that hotter objects radiate much more energy than cooler objects. The second concerns the emission of radiant energy at shorter wavelengths from warmer objects, thereby explaining the difference between the emission of light from the Sun and the emission of heat from Earth.

2.1.2 Solar Radiation is based on the continuous generation of nuclear fusion reactions, the process that converts hydrogen to helium at very high temperatures and pressures.

The energy released by the Sun travels at the speed of light, roughly186000 mi/sec (300000 km/sec). It takes about 8.5 minutes for the solar energy to reach Earth. This energy is mostly in the form of light. The solar constant is the near‐constant rate of radiation that Earth receives from the Sun at the uppermost limits of the atmosphere.

Suggestion: Mention to the class that Earth receives only about one‐half of one‐billionth of the total energy emitted by the Sun. Another very small portion of this energy goes to the other planets and moons in the solar system, while the overwhelming majority travels to the far reaches of outer space.

2.1.3 Characteristics of Solar Energy: The wavelengths emitted from the Sun peak in the visible part of the spectrum, which is why human vision is adjusted to the wavelengths at which the solar light energy is highest. The incoming solar radiation is absorbed and scattered in the process of passing through the molecules and particles in the atmosphere.

2.1.4 Longwave Radiation from Earth occurs at longer wavelengths than the incoming shortwave radiation from the Sun. Water vapor and carbon dioxide in Earth’s atmosphere are major absorbers of longwave radiation and subsequently play a major role in the greenhouse effect.

2.1.5 The Global Radiation Balance is based on the continuous absorption and scattering of incoming solar short-wave radiation and outgoing longwave radiation from Earth’s atmosphere and surface. Over time, Earth’s average temperature remains the same, as the gain and loss of radiant energy maintain a long‐term balance.

2.2 Insolation Over the Globe: Insolation is the flow rate of incoming solar radiation. It is at its maximum when the Sun is at its highest angle in the sky and, as might be expected, at its minimum when the Sun is at a low angle. Note that the higher the Sun is in the sky, the greater the intensity. Conversely, the lower the Sun is in the sky, the lesser the intensity, as the amount of solar energy that is available is spread out over a larger area and is therefore much weaker.

2.2.1 Daily Insolation Through the Year depends on two factors: 1) the angle that the rays of the Sun strike Earth, and 2) the length of time that a particular location is exposed to the Sun’s rays.

Sun Angle Discussion: The angle of the Sun is a function of its path through the day. The Sun is at its highest angle at noon, resulting in maximum insolation. It would be useful to illustrate typical conditions for mid-latitude locations in the northern hemisphere, such as several cities in the U.S. that are close to 40º N Lat., examples of which are New York, Columbus, Ohio, Denver, and Salt Lake City. The Sun’s path for this location at the June solstice attains its maximum height above the horizon at 73.5º at noon when there is 15 hours of daylight. The comparable numbers for the two equinoxes are a maximum height of 50º above the horizon at noon along with 12 hours of daylight. As you might expect, at the December solstice, the Sun provides daylight for only about 9 hours when its noon elevation is only 26.5º above the horizon.

2.2.2 Annual Insolation by Latitude, as expected, reaches a maximum at the Equator and a minimum at the poles. However, the axial inclination of Earth results in a redistribution of incoming solar insolation from the equatorial regions to the poles.

Therefore, the annual insolation of the poles is about 40% of the equatorial value.

Suggestion: The previous paragraph mentioned the importance of Earth’s axial inclination. The instructor may want to repeat the message—the annual insolation at the poles without an axial inclination, i.e. if Earth rotated on a perpendicular axis, would be zero.

2.2.3 World Latitude Zones facilitate the division of Earth into broad but understandable latitudinal zones based on seasonal patterns of daily insolation. An immediate caveat dictates that the following list of zones is generalized; however, these zones are quite useful in understanding regional variation. The zones are as follows: The Equatorial Zone ranges from 10º north to 10º south. Days and nights are of approximate equal length with high insolation throughout the year.

The Tropical Zone ranges from 10º to 25º north and south. This zone experiences high annual insolation with a noticeable seasonal cycle.

The Subtropical Zone ranges from 25º to 35º north and south. This zone experiences high annual insolation in conjunction with a strong seasonal cycle.

The Midlatitude Zone ranges from 35º to 55º north and south. This zone experiences significant variation in the length of daylight from summer to winter, resulting in substantial seasonal contrasts in insolation, commensurate with wide ranges in annual surface temperature.

The Subarctic and Subantarctic zones range from 55º to 60º north and south. The Arctic and Antarctic zones range from 60º to 75º north and south. All these four zones experience very large annual variations in the lengths of daylight, which result in extreme insolation contrasts during the year.

The North and South Polar zones range from 75º to 90º north and south. They experience the greatest contrast in seasonal insolation on the planet, highlighted by 24‐hour periods of total light and darkness for a good portion of the year.

2.3 Composition of the Atmosphere: Earth’s atmosphere is a mixture of many gases held in by gravity that extends to an estimated height of about 6000 mi (10000 km). The bulk of the atmosphere is located within 19 mi (30 km) of Earth’s surface. The overwhelming proportion of atmospheric gases, occurring from the surface to about 50 mi (80 km) in elevation, consists of nitrogen (78% by volume) and oxygen (about 21%). The remaining 1% is mostly argon, an inert gas. Carbon dioxide (CO2) accounts for about 0.0385% of the atmosphere. Although the percentage is very small, CO2 is a particularly important atmospheric gas because it is capable of absorbing not only a large amount of the incoming short-wave radiation from the Sun, but also Earth’s outgoing longwave radiation. Another important component of the atmosphere is water vapor (H2O), which can vary from less than 1% to as much as 2% under warm, moist conditions.

CO2 and water vapor are both very good absorbers of heat radiation; they therefore play a very important role in warming the lower parts of the atmosphere, thereby adding to the greenhouse effect.

2.4 Energy Transfer: Discusses the transfer of sensible and latent heat energy by means of conduction and convection.

2.4.1 Sensible and Latent Heat: Sensible heat is contained within a substance and can be measured by a thermometer. The transfer of sensible heat can occur between the atmosphere and Earth’s surface by conduction when they are in direct contact or by convection when a fluid, such as the atmosphere or ocean, can carry heat energy away from a surface. Latent heat is a form of hidden heat that cannot be measured by a thermometer. It is a type of heat that is carried by changes in the physical state of water.

For example, latent heat transfer would occur when water evaporates from a moist land surface or an area of open water. In the process, heat is being transferred from Earth’s surface to the atmosphere. In short, latent heat is either taken up or released when there is a change of state from a gas to a liquid or ice or vice versa, going from ice to a liquid to a gas.

2.5 The Global Energy System: The flow of energy from the Sun to Earth and then back out into space is a complex system. Solar energy is the ultimate power source for Earth-surface processes, so when we trace the energy flows among the Sun, surface, and atmosphere, we are really studying how these processes are driven.

2.5.1 Solar Energy Losses in the Atmosphere vary substantially from areas with clear skies, which allow more incoming solar radiation to reach the surface, to other places with substantial cloud cover that reflect or absorb larger amounts of solar radiation. As the incoming solar radiation passes through the atmosphere, varying portions of it are absorbed or scattered by large particles, molecules, and dust. Varying amounts of solar radiation are also reflected back to space from land and ocean surfaces and clouds. In an area with clear skies, scattering and diffuse reflection return about 3% of the incoming radiation back to space. Another 17% of the incoming energy can be absorbed by CO2 and water, leaving a potential of 80% to reach the ground. As one might expect, the amount of incoming radiation that is reflected back to space increases substantially with

cloud cover. Clouds can reflect 30‐60% of the incoming energy back to space. Another 5‐20% can be absorbed by clouds. This results in anywhere from 10% to 45% of the incoming radiation actually reaching the ground. On a global scale, it is estimated that only about 50% of the incoming insolation from the Sun that reaches the top of the atmosphere will ever make it to Earth’s surface.

2.5.2 Albedo is the amount of shortwave radiation from the Sun that is reflected back to space by the different surfaces on Earth. The highest albedos, produced by lighter-colored surfaces like snow and ice, range between 45% and 85%. This of course means that most of the incoming radiation is reflected back to space and only a small amount is absorbed. In sharp contrast, roads that are paved with black materials have an albedo of about 3%. This of couse means that most of the radiation is absorbed. It is estimated that overall global albedo is in the range of 29−34%.

Suggestion: It is worthwhile to point out that anthropogenic change to the landscape in itself may affect the various albedos. Ask them what will happen as land surfaces are covered with materials of different reflectivity, as the human population continues to grow and more land is needed for housing, etc. The retreat of glaciers that has been noticed in recent years could also affect the climate in ways not fully understood. Many other examples of substantial change in land use could be considered, but the main purpose in mentioning it here is simply to call attention to a possible problem as changes in albedo rapidly continue.

2.5.3 Counterradiation and the Greenhouse Effect begins with shortwave radiation from the Sun passing through the atmosphere and being absorbed or reflected at Earth’s surface. The surface is warmed by the absorption, resulting in an emission of longwave radiation. As one would expect, some of this reflection goes off to outer space, but another larger portion is absorbed into the atmosphere. The next step is for the atmosphere to radiate some of this longwave energy back to the surface as counterradiation and another portion to outer space. Water vapor and CO2 acts to absorb this counterradiation, thereby producing the greenhouse effect.

Suggestion: Given the ongoing attention to the role of CO2 in climate change, it would be very useful if the class schedule permitted some time to discuss the greenhouse effect and the consequences of its continuation. Suffice it to say that this is more than just an academic exercise.

2.5.4 Global Energy Budgets of the Atmosphere and Surface need to be balanced over the long term. This of course ties in with the attention (and inattention) that a variety of countries at recent international climate-warming meetings have garnered in their attempts to adopt rational procedures to handle global energy budgets.

2.5.5 Climate and Global Change discussion must recognize the importance of human activities that affect energy flows of the Earth‐atmosphere system and must take into consideration the implications of human‐induced changes in shortwave and longwave radiation. Let us also no forget to examine natural Earth cycles and processes and their effects on global warming.

2.6 Net Radiation, Latitude, and the Energy Balance: The balance between incoming and outgoing radiation is referred to as net radiation. At latitudes between 40º N and 40º S, incoming solar short-wave radiation is greater than outgoing longwave radiation, resulting in an energy surplus. The opposite occurs poleward of 40º N and 40º S, where net radiation is negative, resulting in an energy deficit. This imbalance between the latitudinal zones results in major heat transfers from the lower latitudes to the polar regions. These heat transfers include the movement of warm ocean water by major currents (such as the Gulf Stream) and by warm, moist air masses. In turn, cooler water in the form of cold currents (such as the Labrador Current) and cooler, drier air moves equatorward as a major step towards a global balance.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (800‐255‐8629).

Understanding the Greenhouse Effect. DVD DV‐3146, 13 minutes.

What is Weather? DVD DV‐3199, 22 minutes.

Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126).

Protecting Earth’s Atmosphere. DVD PCT‐342‐75‐KS; VHS PCT‐342‐75‐A. 19 minutes, 2004.

Atmosphere, Climate, and Weather. DVD PCT‐347‐27‐K; VHS PCT‐347‐27‐A, approximately 23 minutes, 2006.

Insight Media, New York, NY (800‐233‐9910).

Understanding the Weather. DVD # 79‐AM‐571, 25 minutes, 2000.

Understanding Weather Concepts. DVD # 79‐AS‐1217, 23 minutes, 2002.

Select Reference Books: Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. Pacific Grove, CA: Brooks/Cole, 3rd ed., 2001.

Barry, Roger G. and Richard J. Chorley. Atmosphere, Weather and Climate. New York, NY: Routledge, 8th ed., 2003.

Oliver, John E. (Ed.). Encyclopedia of World Climatology. Dordrecht, The Netherlands, 2005.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 3: Air Temperature Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Identify the factors that affect air temperature.

2) Describe how air temperature varies with altitude.

3) Describe how air temperature varies by time and location.

4) Explain air temperature variations around the world.

5) Describe the consequences of global warming.

This chapter provides details about a concept with which many students have at least a passing familiarity: temperature. However, it introduces many aspects of which students may be entirely unaware. A few current topics, such as the greenhouse effect, global temperatures, and the urban heat island, can provide opportunities to include news stories. This helps to counteract the view held by many students that geography is somehow a slow‐moving and dated subject.

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The Greenhouse Effect: The Greenhouse Effect is a topic that most students have heard about, but many may be unclear on its causes and effects. Sites such as Global Climate Change Student Guide provide general background information, while Global Warming Early Signs may provide some useful topics for debate.

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The Temperature Record: Although students understand the importance of climate change in today’s world, many do not appreciate that climate change is also a natural process that has always been part of Earth’s dynamic evolution. The Climate Time Line site provides a valuable tool to show how pertinent climate change is in our history. The lecture entitled Cultural Responses to Past Climate Change shows how cultures react to climate change. The instructor may have the class develop a list of time periods or historic events and assign students to describe the weather using either of the aforementioned sites.

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The Urban Heat Island: The urban heat island is a good example of how humans can change the environment. Satellite imagery, like that presented on the Remote Sensing of Urban Heat Islands site, also provides useful examples of the role of remote sensing to the investigation of changing climate. The instructor may have the class develop a brief list of cities and assign students to evaluate and compare their heat islands.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are organized for viewing by core concept and by region. (Functioning 10/18/12) GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Today from Space—NASA. This site provides real-time images and data on current temperature and weather as well as up-to-date data on global temperature changes. http://www.nasa.gov/audience/foreducators/son/winter/weather_watch/stations/F_Weather_Watch_ NWS.html Climate Time Line Information Tool. This site documents climate change. http://www.ngdc.noaa.gov/paleo/ctl/ Nova Warnings from the Ice. A website that describes the types of information that can be obtained from cores taken from today’s ice sheets. Useful in explaining what this information tells us about how the climate operates. http://www.pbs.org/wgbh/nova/warnings/ Nova and Frontline What’s Up with the Weather? This website examines recent apparent increases in extreme and unseasonable weather. http://www.pbs.org/wgbh/warming/ The Ancient Bristlecone Pine. Website focusing on the use of this long-lived tree species to reconstruct proxy climate data. http://www.sonic.net/bristlecone/home.html Climate Change Notes and web links from a meteorology course at Lyndon State College. http://apollo.lsc.vsc.edu/classes/met130/notes/chapter18/index.html Heat Island Effect. EPA website that explains the urban heat island effect. http://www.epa.gov/hiri/ Remote Sensing of Urban Heat Islands. This resource includes an exercise to detect heat islands from a clickable satellite image. http://itg1.meteor.wisc.edu/wxwise/museum/a3/a3heatisl.html

Chapter Lecture: 3.1 Surface and Air Temperature: A weather station’s air temperature and its variations are heavily influenced by five important factors: latitude, surface type (topography), coastal or interior location, elevation, and atmospheric and oceanic circulation.

3.1.1 Surface Temperature provides a measure of the flow of radiation into or out of the surface of a substance such as a gas, liquid, or solid. Net radiation represents the balance between incoming shortwave radiation and outgoing longwave radiation, thereby either heating or cooling a surface. As might be expected, the net radiation balance is positive during the day and negative at night. The movement of sensible heat from a warmer

substance to a colder one by direct contact is referred to as conduction. The evaporation or condensation of water as it changes its state from vapor to liquid is called latent heat transfer and can cool or heat the surface. Convection is another form of energy transfer in which heat is mixed in a fluid.

3.1.2 Air Temperature is measured in a white, louvered box called a Stevenson shelter at a standard height of 4 ft (1.2 m) above the ground. Typical observations for a weather station include the minimum and maximum for a 24‐hour period. The mean daily temperature is then easily calculated as the average of the minimum and maximum temperatures for the recording period.

Suggestion 1: Mention to the class that the purpose of the white color of the Stevenson box is to raise the albedo in order to reduce possible warming during the day. The louvers in the box provide ventilation for the instruments so that air can flow in and out.

Suggestion 2: Students in the class who are weather buffs can check with their college libraries to see if copies of monthly climatological data are available for their state. These reports are issued by the National Climatic Data Center in Asheville, NC and are also available for purchase. Anyone using these reports should also know that different weather stations use varying 24‐hour periods for their observations. For example, some take their readings at 7:00 a.m. or 8:00 a.m., while others, particularly at airports, record their measurements at midnight. As a result, daily observations may reflect differences among the weather stations, particularly in amounts of precipitation for a 24‐hour period.

3.1.3 Temperatures Close to the Ground indicate that air and soil temperatures taken at the ground surface, or very close to the ground, are much more variable than the observations made at the standard above-ground height of 4 ft (1.2 m).

3.1.4 Environmental Contrasts: Urban and Rural Temperatures refer to the substantial differences between urban and rural surfaces. Urban surfaces generally lack water due to an extensive network of storm drains that collect the excess water and remove it from the streets, thereby slowing or preventing evaporation, which is a cooling process. During the evening hours, urban pavements can conduct the stored heat in the ground to the surface, which leads to warmer temperatures.

3.1.5 The Urban Heat Island is caused by the large difference between urban and rural surfaces that makes densely settled places warmer during the day and night. The heavy use of air conditioning in towns and cities during the warmer months also adds to the heat-island effect.

Green Roof Comments refer to the growing interest, where possible, in adding plants and trees to roofs on office buildings as a way of reducing the large difference between urban and rural areas in terms of heat entrapment. The cooling effects of evapotranspiration (the combined processes of evaporation and transpiration) by plants can help lower temperatures and the magnitude of the heat island effect.

3.1.6 High‐Mountain Environments have simply less air to absorb solar radiation.

Consequently, places at high altitudes have both generally cooler temperatures and also a larger diurnal range. This occurrence is a reflection of the diminished thickness and

density of the air as one ascends into higher altitudes. Accordingly, the greenhouse effect is simply weaker at higher elevations.

3.1.7 Temperature Inversions occur when air temperatures increase with altitude, rather than decrease (which is the normal pattern). This situation is more likely on calm, clear nights when the surface can lose longwave radiation to space. This situation can result in killing frosts that can seriously damage crops, such as oranges, that are very sensitive to cold temperatures.

3.1.8 Temperature Indexes provide useful environmental indicators of how warm or cold it feels to humans. For example, the wind chill index is based on the actual temperature and the wind speed outdoors and provides a much more realistic measure of how cold we would actually feel if we went outside. It is quite common to hear radio and TV weather announcers talk about the effects of the wind chill factor. The heat index is based on the actual temperature and relative humidity and provides a useful indicator of how hot the air feels. As higher moisture levels in the atmosphere reduce the amount of perspiration that can be evaporated from our skin, the body loses its ability to cool itself.

3.2 Temperature Structure of the Atmosphere: The bulk of incoming solar radiation travels through the atmosphere until it is absorbed at Earth’s surface. In turn, latent and sensible heat flows from the surface to warm the atmosphere. It is reasonable, then, to expect that the farther you are from the surface, the cooler the temperature. The temperature decrease with altitude is called the environmental temperature lapse rate and has a value of 3.56°F/1,000 ft (6.49°C/1,000 m).

Suggestion: It would be useful to remind the class that this lapse rate can and does vary from the aforementioned values. It is not a fixed value. Its changes have implications that will be discussed later in the book.

3.2.1 The Troposphere is the lowest layer in the atmosphere. Its thickest portion (10 mi; 16 km) is located in the equatorial and tropical regions, and its thinnest section (4 mi; 6 km) is found at the poles. The abundance of very small particles called aerosols is very important as water vapor can condense on them and form tiny droplets. These droplets can grow larger and become visible as fog and clouds of various types.

3.2.2 Stratosphere and Upper Layers: The tropopause marks the boundary between the troposphere and the stratosphere and extends to roughly 30 mi (50 km) above Earth’s surface. It is characterized by strong persistent winds that flow from west to east. The stratosphere generally holds minimal amounts of water vapor or dust. The mesosphere and the thermosphere form the next two higher layers of the atmosphere. The homosphere extends from Earth’s surface to about 62 mi (100 km) and, as the name suggests, has a uniform composition.

Ozone Layer Comment: This is an important topic to discuss with the class, as it is a problem that began in the late 20th century. To begin with, there is a layer in the stratosphere that contains a small, but very important, constituent known as chlorofluorcarbons or CFCs. They are synthetic industrial chemical compounds that were used in aerosol sprays and as cooling fluids in refrigerating and air-conditioning systems. As these CFCs were released into the atmosphere, they reduced the ozone concentration within the ozone layer, thereby decreasing its ability to absorb ultraviolet energy. This ozone diminishment can lead to an increase in skin cancer cases, crop yield reduction,

and harm to some forms of aquatic life, as more ultraviolet energy can get through to Earth’s surface. It is hoped that international pressure will encourage all countries to ban the use of CFCs and turn towards less harmful refrigerants.

3.3 Daily and Annual Cycles of Air Temperature: The amount of insolation that Earth receives depends on the latitude and the season. Therefore, net radiation, as one might expect, is positive during the day and negative at night. The peak time of insolation throughout the year is at noon, but the actual amount varies with the seasons. Throughout the year, the minimum daily temperature takes place about 1/2 hour after sunrise, based on the net negative radiation during the night that has cooled the surface air temperature to a minimum. Due to convection currents that develop on sunny days relatively close to the surface, air temperature generally peaks 2−4 hours after 12 noon.

3.3.1 Land and Water Contrasts strongly affect daily and annual temperature cycles. Since water bodies heat and cool much more slowly than land surfaces, maritime locations have more moderate temperatures throughout the year when compared to the much larger spread of temperatures for inland continental locations.

Suggestion: This is a good place to mention again the fivefold difference between the specific heat of dry land (0.2) and water (1.0). This difference in specific heat also explains the lag in average minimum and maximum temperatures of 1−2 months after the winter and summer solstice.

3.3.2 Annual Net Radiation and Temperature Cycles reflect the seasonal variation of insolation that drives the annual cycle of net radiation. These factors strongly influence the annual cycle of air temperatures. In those instances where there is a strong seasonal variation with net radiation, surface air temperatures will also have a marked seasonal variation. Thus, Patone would expect annual temperature ranges to increase as latitude increases from the equator to the poles. Thus, equatorial temperatures would have minimal seasonal variation compared to poleward locations.

3.4 World Patterns of Air Temperature: An isopleth is an imaginary line on a map that connects points having the same value. A family of isopleths can also show gradients or changes in value between different locations on the map. In the case at hand, an isotherm by definition connects points that all have the same temperature.

Suggestion: This is a good opportunity to mention other isopleths that are germane to physical geography. A sample list would include isobars (barometric pressure), isobaths (bathymetric depths for lakes and oceans), and isohyets (precipitation values).

3.4.1 Factors Controlling Air Temperature Patterns, based on world patterns of isotherms, include: 1) Latitude—average annual insolation decreases as the latitude increases, 2) Coastal‐Interior Contrasts—oceanic air and currents result in more uniform annual temperatures in coastal locations, as compared to the much greater variation in temperatures for interior locations, 3) Elevation—temperatures are lower in mountainous areas.

3.4.2 World Air Temperature Patterns for January and July: Large land masses in the northern parts of Eurasia and North America have extremely low winter temperatures due to their limited insolation during long periods of the year and the high albedo of the snow that reflects a large portion of the incoming solar insolation back to outer space. There is a temperature decrease from the Equator to the poles that is in response to the reduction in

insolation as the latitude increases. Areas of continuous ice and snow, such as the ice sheets in Greenland and Antarctica, are always extremely cold. The causal factors include high elevations of the central portions of the ice sheets that top 10,000 ft (3,000 m) and the high albedo of the snow that reflects a large portion of the incoming solar insolation back to outer space. Mountain areas are always colder that their surrounding lowlands because temperatures decrease as elevations increase. There is minimal change in temperatures from January to July in equatorial areas since they experience only slight variations in insolation.

Accordingly, the temperatures are uniform throughout the year. There is a large north‐south latitudinal shift from January to July in continental isotherms in the midlatitudes and subarctic land areas. The comparable shift in isotherms over the oceans is muted, as continents heat and cool much faster than oceans.

3.5 The Temperature Record and Global Warming: There is increasing evidence from many climatic records that global temperatures have been increasing over the past several decades. The carbon dioxide (CO2) that is released by the burning of fossil fuels is one key factor in the warming process, but other greenhouse gases such as methane (CH4), chlorofluorocarbons (CFCs), ozone (O3), and nitrous oxide (NO3) also play an important role. Factors influencing climatic warming and cooling include several important gases, human activities, and naturally occurring events. The greenhouse gases generally work together to increase global warming as compared to aerosols, cloud changes, and human‐induced land‐cover alterations that lead to global cooling. Some natural factors, such as solar output, increase global warming slightly, whereas emission of volcanic aerosols can have warming or cooling effects. The greenhouse gases are key elements in global warming, the most important of which is CO2. However, gases such as methane (CH4), chlorofluorocarbons (CFCs), ozone (O3), and nitrous oxide (NO3) also play a key role. Currently, the warming effect of greenhouse gases exceeds the cooling effects of the other factors.

3.5.1The Temperature Record for the world shows a rising trend for the past 50 years. This warming trend continues even though volcanic eruptions eject particles and gases into the stratosphere, allowing the aerosols that reflect solar radiation to spread rapidly and thereby have a cooling effect on the planet. Temperature reconstruction refers to techniques that can be used to extend the thermal record back by several centuries or more. These indirect methods include tree‐ring, coral, and ice-core analyses.

3.5.2 Consequences of Global Warming include: 1) Change in global surface temperature, 2) Melting of polar sea ice, 3) Global sea-level rise, 4) Arctic thawing, and 5) Habitat loss.

Future scenarios use computer climate models to see if the present trend of rising temperatures will continue into the future. A warming climate could melt glaciers and sea ice, thereby increasing sea levels and having a drastic effect on coastline populations.

Climate change could also promote the spread of insect‐borne diseases such as malaria. As if this wasn’t enough, Earth’s climate could become that much more variable. It has already been noted that storms have increased in frequency since 1980.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (800‐255‐8629). What Makes Weather. DVD DV‐3202; VHS VC‐3315, 14 minutes.

Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126). Meltdown: A Global Warming Journey. DVD OVW‐359‐91‐KS; VHS OVW‐355‐91‐A. 60 minutes, 2006. Too Hot Not to Handle: Winning the Battle Against Global Warming. DVD OVW‐362‐44‐S, 55 minutes, 2006.

Insight Media, New York, NY (800‐233‐9910): Core Meteorology: Atmosphere. DVD 8‐AS‐2475, 30 minutes, 2008. Core Meteorology: Weather. DVD 8‐AS‐2499, 30 minutes, 2008.

Select Reference Books: Coley, David. Energy and Climate Change. Hoboken, NJ: Wiley, 2008.

Gautier, Catherine. Oil, Water, and Climate. New York, NY: Cambridge University Press, 2008.

Lynas, Mark. High Tide: The Truth About Our Climate Crisis. New York, NY: Picador, 2004.

Tickell, Oliver. Kyoto2: How to Manage the Global Greenhouse. New York, NY: Zed Books, 2008.

Wolfson, Richard. Energy, Environment, and Climate. New York, NY: W.W. Norton, 2008.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 4: Atmospheric Moisture and Precipitation Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe the hydrologic cycle.

2) Compare specific and relative humidity.

3) Compare the dry and moist adiabatic rates.

4) Describe the forms and behavior of clouds.

5) Explain how precipitation forms.

6) Identify different types of precipitation.

7) Describe the causes and types of thunderstorms.

8) Describe the characteristics and development of tornadoes.

9) Identify factors that reduce air quality.

The main focus of this chapter is to examine the various aspects of atmospheric moisture and the process of precipitation. It is crucial to discuss the role of temperature in producing precipitation, as moist air must be cooled before condensation can form water droplets that result in precipitation.

Related topics in the chapter include acid deposition and air quality. This chapter also presents an array of topics from concepts such as humidity and lapse rates to weather features ranging from clouds and fog to thunderstorms and air pollution.

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The Hydrological Cycle: The hydrological cycle has many familiar features (for example, water evaporating from the ocean surface and finding its way into the atmosphere). Point students to a brief introduction to the hydrological cycle and develop some challenging questions that can be answered there.

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Lapse Rates: Lapse rates are one of the more difficult topics for many students to grasp, since it can be difficult to envision the processes involved. The lecture on lapse rates, moisture, clouds, and thunderstorms can help students review the topic. The animations in WileyPLUS could be used in class to describe these processes as well as a review for students as they study.

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Cloud Types and Fog: Clouds are such a familiar feature that they are often ignored; students are often unaware of the great range of cloud structures and types. The Cloud Boutique, Clouds and Precipitation, and Cloudman provide many images of cloud types to which students can refer. The interactivity in WileyPLUS has a number of satellite images that students can label in terms of cloud types and fog. Although clouds are familiar, many students do not realize that we still have much to learn about them. The International Satellite Cloud Climatology Project site is a good example of an extensive cloud-research project.

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Precipitation and Humidity: Humidity is familiar to students because it is usually mentioned in weather forecasts. Students can be encouraged to develop a more detailed understanding by first reviewing this topic at the Meteorology On-line site and then undertaking some simple calculations at the Humidity Calculator site.

Thunderstorms: Severe weather, such as thunderstorms, usually generates a great deal of interest. The sites provided here include an array of impressive images as well as current research. A useful way to develop enthusiasm is to include examples that apply to student’s everyday lives. The National Weather Warnings site provides valuable information concerning extreme weather that everyone should find useful.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on the Earth's surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are organized for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Humidity from Meteorology On-line. A review of humidity with practical background information. http://library.thinkquest.org/C0112425/stu_rain.htm Snow Crystals. Here you can learn about the ever-varied and intricate patterns of snow crystals and how they form. http://www.its.caltech.edu/%7Eatomic/snowcrystals/ The Aurora Page. Although the textbook does not cover auroras, they are also interesting phenomena to investigate. Often people do not notice the Northern Lights because they do not know what to look for. Learn about them here. http://www.geo.mtu.edu/weather/aurora/ Cloud Boutique from the Plymouth State College Meteorology Program. This site provides an extensive collection of cloud images. http://vortex.plymouth.edu/clouds.html Clouds and Precipitation from the On-line Weather Guide. This site reviews the different concepts associated with clouds and precipitation. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/home.rxml Cloudman is a web site devoted to observing clouds. Included are a cloud atlas, cloud gallery, and tips on photographing clouds. http://www.cloudman.com/ International Satellite Cloud Climatology Project (ISCCP). A global project to provide synchronous estimates of cloud cover across Earth. http://isccp.giss.nasa.gov/ NOAA’s Drought Site. This website monitors and evaluates drought conditions and includes the Drought Calculator, which can assess how much rain would be required in a region to end a drought. http://www.drought.noaa.gov/

Storm Prediction from the National Weather Service. Provides real-time data on current weather watches, thunderstorms, and fire risk. http://www.spc.noaa.gov/products/index.html The Disaster Center Lightning page. Find out more about current research on lightning, including technologies to protect properties as well as some amazing images. http://www.disastercenter.com/lightng.htm National Weather Warnings for Extreme Weather Events in the USA. Examine the system that categorizes the various forms of extreme weather warnings, a topic about which everyone should be informed. http://iwin.nws.noaa.gov/iwin/nationalwarnings.html Chapter Lecture 4.1 Water in the Environment: Our planet’s surface is dominated by water. Over 70% of Earth’s surface is covered by liquid water, in oceans and lakes, and solid water, in the ice of glaciers, icecaps, and sea ice. In this chapter, we focus on water in the air, both as vapor and as liquid, and on solid forms of H2O. Precipitation is the fall of liquid or solid water from the atmosphere that reaches Earth’s surface. It forms when moist air is cooled, causing water vapor to form liquid droplets or solid ice particles. If cooling is sufficient, liquid and solid water particles will grow too large to be held aloft by the motion of the atmosphere and will fall toward the surface.

4.1.1 Properties of Water: The water molecule (H2O) is a chemical compound of hydrogen and oxygen, two very abundant elements. The two hydrogen atoms share electrons with the one oxygen atom in what chemists refer to as a covalent bond. The arrangement of the three atoms produces a molecule with a weak positive charge on one side and a weak negative charge on the other. Since positive and negative charges attract, liquid water molecules tend to stick together, negative to positive. This weak attraction is called hydrogen bonding. It also helps water molecules form thin films on solid substances, adhering to natural positive and negative charges on substance surfaces. The attraction between water molecules produces surface tension, forming a thin skin of surface water molecules that makes a water droplet form a bead.

When surface tension is coupled with adherence to a solid surface, thin films of water can be drawn into fine cracks and openings, such as small openings in rocks and soil. This is known as capillary action or capillarity.

4.1.2 Three States of Water. There are many interesting physical aspects about water. It is an odorless, tasteless, and transparent compound that is critical in all biological, physical, and chemical processes. With the possible exception of what may be ice found in certain polar locations on Mars and the Moon, water is found in great abundance on only one planet in our solar system: Earth. Indeed, water covers about 70% of Earth’s surface in its solid and liquid forms.

Another unusual property of water is its very high heat capacity, which, in turn, is associated with the latent heat of fusion and vaporization. The latent heat of fusion is the amount of heat per unit mass (80 calories/gram) that is required to completely change a substance at its melting point to a liquid at the same temperature. In other words, if heat is applied to ice at 32º F (0º C), the temperature of the ice remains constant until all of the ice has melted. In a similar manner, the latent heat of vaporization is the amount of heat per unit mass (540 calories/gram) that is required to change a liquid at its boiling point to a gas at the same temperature. As in the

previous example, this means that if heat is applied to water at 212º F (100º C), the water will start to boil and the temperature will stay the same until there is no liquid water left. Note that the term “latent” is indicative of a change of state without a change in temperature.

Suggestion: Mention to the class that the old saying “A watched pot never boils” simply reflects the fact that water needs a huge amount of heat (and therefore time) before it can reach its boiling point and thereby undergo a phase change from liquid to vapor.

Sublimation occurs when ice changes from a solid to a vapor form. The reverse process from a vapor form to a solid form is known as deposition.

Suggestion: Mention to the class (particularly those in colder climates) that sublimation can be witnessed when ice or snow on a sidewalk or road is seen at one time in its solid form and then disappears at a later time without leaving a water trail. The reverse process (deposition) is easily witnessed when ice forms on a car windshield at night and then disappears later on during the day without leaving a water trail.

4.1.3 The Hydrosphere. The realm of water in all its forms, and the flows of water among ocean, land, and atmosphere, are known collectively as the hydrosphere. About 97.2% of the hydrosphere consists of marine saltwater. The remaining 2.8% is freshwater. The next largest reservoir is freshwater stored as ice in the world’s ice sheets and mountain glaciers, accounting for 2.15% of total global water. This means that almost all of the water on Earth (99.35%) is either salty or frozen. The bulk of the remaining 0.65% of the planet’s water is groundwater (0.63%). Water in the atmosphere accounts for only 0.001% of the total amount.

Interestingly, the smallest fraction of the world’s water (0.0001%) is found in the combined average flows of the all of the rivers on Earth, ranging from minor streams to the mighty Amazon.

Suggestion 1: Comment to the class that, although Earth has large expanses of deserts (the Sahara, Gobi, etc.), it is well watered. For example, if we assume that the entire surface of this planet were leveled off and the oceans filled with this continental material, Earth would be hypothetically covered with water to a depth of more than 1.9 miles (3 km). Now you can lead into the fact that most of Earth’s water (97.2%) is contained in the oceans. Another 2.15% is locked up in ice caps and glaciers.

Suggestion 2: Discuss with the class the problem of water held in its frozen form and what is/will happen if global temperatures continue to rise. Mention sea level rise and its impact on coastal communities.

4.1.4 The Hydrologic Cycle, or water cycle, moves water from land and ocean to the atmosphere. Water from the oceans and land surfaces evaporates, changing state from liquid to vapor and entering the atmosphere. Total evaporation is about six times greater over oceans than land because oceans cover most of the planet and because land surfaces are not always wet enough to yield much water. The amount of water on Earth is relatively fixed. Some water molecules can escape from the uppermost part of the atmosphere and volcanic eruptions can

release some water from the deepest parts of Earth; however, the quantities are very small.

Essentially, the planet cycles an enormous amount of water in its three states, although its availability varies substantially on land.

Suggestion: Begin this section by stating that water is a renewable resource and that the hydrologic cycle involves the global flow of water to and from the land, sea, and atmosphere. For example, the water that was drunk by Caesar or Cleopatra in ancient times was released from their bodies and then entered the ground or a stream or the ocean where it eventually evaporated back into the atmosphere and came back to Earth as some form of precipitation.

4.2 Humidity: The amount of water vapor that is in the air is called humidity. Although it makes up only a very small fraction of the atmosphere (0.1 to 4%), its importance and spatial variation are enormous. Indeed, the amount of water vapor in the air can be considered the most important gas in the atmosphere in terms of atmospheric processes.

4.2.1 Specific Humidity: The actual amount of water vapor in the air at a certain location is called specific humidity and is measured as so many grams of water vapor per kilogram of air (g/kg). The global variation is enormous, ranging nearly two orders of magnitude from 0.2 g/kg in very cold, dry air over the polar regions to 18 g/kg over equatorial regions.

4.2.3 Dew-Point Temperature of a parcel of moist air is the temperature at which the air becomes saturated. The more water vapor in the air, the higher the dew-point temperature.

4.2.4 Relative Humidity measures water vapor in the air as the percentage of the maximum amount of water vapor that can be held at the given air temperature. Relative humidity is expressed as a percentage. If it is 100%, then the air is holding the maximum amount of moisture that is possible at that temperature.

Suggestion 1: Begin with some easily observed aspects of low and high relative humidity. For example, you can start with piano tuning. Low and high relative humidity affect the wood in the piano and result in subtle (and in some cases not so subtle) changes in tone. Another example can be the drying of skin when the relative humidity is too low and the general discomfort if it is too high. Note how we need dehumidifers in the summer and humidifiers in the winter (in the northern hemisphere, particularly when forced hot-air heating is used).

Suggestion 2: If your department has some modest funds ($65-$75), purchase a sling psychrometer and show the class how it works (a photo is available in the text). Have a student write on the board in a simple two-column table how the difference between the wet and dry thermometer changes as one keeps swinging the instrument. Then let the class see what happens as the wet thermometer slowly dries to the point where the two thermometers have the same temperature.

4.3 The Adiabatic Process. The adiabatic principle states that when a gas is compressed, it warms, and when a gas expands, it cools. When an air parcel moves upward in the atmosphere, it encounters a lower pressure and so expands and cools. If air is compressed, it will be warmed. Conversely, if it is allowed to expand, it will be cooled. Essentially, it is the change in pressure that affects the temperature of a gas rather than an inflow or outflow of heat energy.

Suggestion: Ask students who have bikes what they feel on their (ungloved) palms when they manually pump air into their tires (compression: warmth) as compared to what they feel if they release air from the tire (expansion: cold air).

4.3.1 Dry Adiabatic Lapse Rate. As air is lifted (by mechanisms to be explained shortly), the atmospheric pressure is reduced and the air parcel expands and cools. Conversely, if an air parcel descends, the atmospheric pressure increases and the air is compressed and warmed.

Since condensation has not occurred in this example, the process is referred to as the dry adiabatic rate, which is 5.5º F/1,000 ft (10º C/1,000 m).

Suggestion: Let the class know that various lapse rates will be discussed in this chapter, and the only one that remains the same is the Dry Adiabatic Lapse Rate.

4.3.2 Moist Adiabatic Rate. As a parcel of air rises, condensation begins to occur when the air is cooled to its dew-point temperature. Further uplift and cooling allows water droplets to form into clouds. The release of latent heat allows the cooling process to proceed at a reduced rate.

The wet adiabatic rate represents the cooling rate for saturated air, which varies between 2.2 and 4.9º F/1,000 ft (4 and 9º C/km).

Suggestion: Tell the class to look out for fair-weather cumulus clouds that appear to be at the same elevation in the sky. The base of these clouds represents the altitude at which the adiabatic rates have changed from dry to wet and condensation now occurs.

4.4 Clouds and Fog: There are many different types of clouds, but they all consist of water droplets, ice particles, or some combination of both that is suspended in the air. These cloud particles are very small and grow on even smaller forms of condensation nuclei, such as minuscule specks of sea salt and dust.

Suggestion: Ask the class to think about the effect of industrial emissions on cloud formation. Indeed, the next question would be to consider the effect on the atmosphere from the formation of contrails from jet planes at high altitudes.

4.4.1 Cloud Formation. Cloud particles grow around a tiny center of solid matter. This dust speck of matter, called a condensation nucleus, typically has a diameter between 0.1 and 1 micrometers (0.000004 and 0.00004 in.).

4.4.2 Cloud Forms. Clouds can be classified into four families based on height: low, middle, high, and those that are characterized by vertical development. Clouds are also grouped into two major classes based on form: stratiform for layered clouds and cumuliform for clouds that have vertical development and are rising because they are warmer than the surrounding air.

4.4.3 Fog. Fog is basically a cloud layer that is very close to Earth’s surface. One type of fog, known as radiation fog, forms at night when the temperature of the air layer at ground level falls below the dew point. This kind of fog forms in valleys and low-lying areas, particularly on clear winter nights when radiative cooling is very strong.

Suggestion: This is a good opportunity to bring up the topic of damage to orchards in central Florida and California, where valuable crops like oranges and grapefruit can be seriously affected. Interesting solutions to this problem have included the following: 1) the use of fuel-burning ground heaters that are effective but expensive becuse you need about 30-40/acre (74-99/hectare), 2) the use of stationary wind machines with propellers that can mix warmer air aloft with cooler air closer to the surface, and 3) the sprinkling of the fruit with water so that it can freeze on the outside and protect the inside from further damage due to the latent heat of fusion being released.

4.5 Precipitation: Precipitation forms when water droplets in warm clouds condense, collide, and coalesce into droplets that are large enough to fall as rain. In cold clouds, ice crystals grow by deposition, while water droplets shrink by evaporation. There are four types of precipitation processes: orographic, convective, cyclonic, and convergent.

4.5.1 Formation of Precipitation. Clouds are the source of precipitation for planet Earth. Small water droplets in warm clouds can condense, collide, and finally combine into larger droplets that can then fall as rain. If the clouds are colder and thereby contain a mixture of ice crystals and water droplets, ice crystals can form. If they become large enough, they can fall from the cloud as snow. In addition to rain and snow, precipitation can also occur as sleet and freezing rain, and as hail, which forms in the strong updrafts of thunderstorms where ice layers can accumulate on ice pellets until they reach a size and weight that forces them to fall to Earth.

Suggestion: Mention the fact that some farmers buy hail damage insurance as a means of coping with the partial or full loss of their crop.

4.5.2 Precipitation Processes. In order to precipitate, air has to ascend and be chilled at the dry adiabatic lapse rate to the point of saturation and then continue to move upward at the moist adiabatic lapse rate, where condensation occurs that can eventually lead to precipitation. What is needed in this process is something to force the air upward. Nature provides four modes of uplift: 1) orographic, 2) convective, 3) cyclonic, and 4) convergent. The first two are covered in this chapter; the remaining two are discussed in Chapter 6.

4.5.3 Orographic Precipitation. This type of precipitation is caused by air that is forced to rise over a mountain boundary. As the air moves up on the windward side of the mountain, it is cooled at the dry adiabatic lapse rate. At some point up the mountain, condensation occurs and clouds form. Cooling then proceeds at the moist adiabatic lapse rate and precipitation begins.

After reaching the summit of the mountain, the air starts descending and warming at the dry adiabatic lapse rate on the leeward slope.

Suggestion: California provides an excellent example of this type of precipitation. If possible, suggest to the class that they could witness the effects of orographic precipitation on the landscape if they ever travel between Sacramento and Reno along I-80. This 140-mile trip should, of course, be done during the day in order to see the transition as you start in the Central Valley and go up through the foothills of the Sierras to Donner Summit at elevation 7,239 ft (2,206 m). The tall trees and thick vegetation at this point indicate abundant precipitation. Note that the peaks of the Sierras reach 14,000 ft (4,200 m) and are nearly twice the elevation of I-80 at the Donner Pass. The highway begins to descend as you approach the Nevada border, and the thinning of the trees and eventual shift in vegetation become

apparent as you approach Reno at an elevation of 4,490 ft (1,369 m). You are now on the leeward side of the Sierras, and the air is descending at the dry adiabatic lapse rate.

4.5.4 Convective Precipitation. Unequal heating at the ground surface provides an excellent opportunity for this type of precipitation to occur. Air can rise in a convection cell over warmer portions of a landscape because it is less dense than the surrounding air. As the cell rises, it cools at the dry adiabatic lapse rate. Further cooling from continued uplift allows condensation to occur.

Suggestion: An easy way to have the class get a feel for unequal heating at the ground surface is the simple barefoot test, best done on a warm to hot summer day. Obviously, this is an out-of-class experiment. Have some willing volunteers remove their shoes and socks and walk across a narrow street that is paved with blacktop, then move over to a sidewalk of gray or white concrete, and finally end on a green lawn. The difference in heat among these three surfaces should give them a quick understanding of how convective precipitation can occur.

4.5.5 Unstable Air. This type of air is warm, wet, and when heated by the ground surface, can develop into prominent cumulonimbus clouds that can result in abundant convective precipitation. This situation is quite common in the central and southeastern United States during the summer when hot, moist, air masses are abundant. Three environmental conditions govern what occurs: 1) If the environmental lapse rate is less than the dry adiabatic lapse rate and the moist adiabatic lapse rate, stability will occur.

2) If the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate, then the atmosphere at this point is conditionally stable; however, stability depends on whether or not the ascending air parcel is unsaturated or saturated.

3) If the environmental lapse rate is greater than both the dry adiabatic lapse rate and the moist adiabatic lapse rate, then the atmosphere is unstable. This means that the air will continue to rise to even higher levels.

4.6 Types of Precipitation: Precipitation consists of liquid water drops and solid crystals that fall from the atmosphere and reach the ground. This precipitation can take several different forms: rain, snow, sleet, freezing rain, and hail.

4.6.1 Rain. Rain is precipitation that reaches the ground as liquid water. Raindrops can form in warm clouds as liquid water, which through collisions can coalesce with other drops and grow large enough to fall to Earth. They can form through other processes as well. For example, solid ice, in the form of snow or hail, can also produce rain if it falls through a layer of warm air and melts along the way.

4.6.2 Snow. Snow forms as individual water vapor molecules are deposited on existing ice crystals. If these ice crystals are formed by deposition, they take the shape of snowflakes, with their characteristic intricate crystal structure. However, most particles of snow have endured collisions and coalesce with each other and with supercooled water drops. As they do so, they lose their shape and can become simple lumps of ice. Eventually, whether they are intricate snowflakes or accumulations of ice and supercooled water drops, these ice crystals become heavy enough to fall from the cloud. If the underlying air layer is below freezing, snow produced in cold clouds reaches the ground as a solid form of precipitation; otherwise, the snow melts and arrives as rain.

4.6.3 Sleet and Freezing Rain. When snow falls into a warm air layer at the ground, the melting ice particles are called sleet. Perhaps you have experienced an ice storm. Ice storms occur when the ground is frozen and the temperature of the lowest air layer is also below freezing.

Actually, ice storms are more accurately named “icing” storms because it is not ice that falls but freezing rain—supercooled drops that freeze on contact. The freezing rain creates a clear, slippery glaze on roads and sidewalks, making them extremely hazardous. Ice storms cause great damage, especially to telephone and power lines and to tree limbs, which are pulled down by the weight of the ice.

4.6.4 Hail. Hailstones are formed by the accumulation of ice layers on ice pellets that are suspended in the strong updrafts of thunderstorms. As these ice pellets—called graupel—move through subfreezing regions of the atmosphere, they come into contact with supercooled liquid water droplets, which subsequently freeze to the pellets in a thin sheet. This process, called accretion, results in a buildup of concentric layers of ice around each pellet, giving it its typical ball-like shape. With each new layer, the ball of ice—now called hail—gets larger and heavier.

When it becomes too heavy for the updraft to support, it falls to Earth. When the updrafts are extremely strong, the hail remains aloft, slowly accumulating more mass and getting larger. In that case, hailstones can reach diameters of 3 to 5 cm (1.2 to 2.0 in).

4.6.5 Measuring Precipitation. Precipitation is recorded as a depth that falls during a certain time—for example, as millimeters or inches per hour or per day. A millimeter of rainfall would cover the ground to a depth of 1 mm if the water did not run off or sink into the soil. Rainfall is measured with a rain gauge. This simple meteorological instrument is constructed from a narrow cylinder with a wide funnel at the top. The funnel gathers rain from a wider area than the mouth of the cylinder, so the cylinder fills more quickly. The water level gives the amount of precipitation, which is read on a graduated scale. For meteorological records, snowfall is measured by the amount of liquid water it yields when melted. We can also measure snowfall by depth in millimeters or inches. Ordinarily, a 10-mm (or 10-in) layer of snow is assumed to be equivalent to 1 mm (1 in) of rainfall, but this ratio may range from 30 to 1 in very loose snow to 2 to 1 in old, partially melted snow.

4.7 Thunderstorms: Any storm that can produce thunder and lightning is called a thunderstorm. It is characterized by strong updrafts of air within billowing cumulonimbus clouds. In addition to heavy rain, hail and lightning are also common. Thunderstorms can result in extensive damage to structures and crops.

4.7.1 Air-Mass Thunderstorms. Air-mass thunderstorms are isolated thunderstorms generated by daytime heating of the land surface. They occur in warm, moist air that is often of maritime origin. Triggered by solar heating of the land, they start, mature, and dissipate within an hour or two. Formation stops at night, since surface heating is no longer present.

4.7.2 Lightning. Lightning is a giant electric arc passing through the atmosphere, and is also generated by convection cell activity. It occurs when updrafts and downdrafts cause positive and negative static charges to build up within different regions of the cloud. The exact mechanism is not completely understood, but generally, negative charges are carried downward and positive charges are carried upward. When the separation of charges reaches a threshold, the molecules and atoms of the atmosphere become conductive, and electric current flows along a narrow path. The atmosphere is heated explosively to produce light and generate a shock wave in the air, which we hear as thunder.

4.7.3 Severe Thunderstorms. Severe thunderstorms persist longer than air-mass thunderstorms and have higher winds. They often produce hail or even tornadoes. Although they may start as simple air-mass thunderstorms, they reach a mature stage and then intensify rather than dissipate. In the severe thunderstorm, large amounts of cooler, drier environmental air enter the cloud from the upwind side, creating a strong downdraft. As the downdraft spreads out in front of the storm, it creates a gust front. The advancing air pushes large volumes of moist surface air upward, feeding the convection. A distinctive roll cloud can form that is visible as the storm approaches.

Wind Shear—a change in wind velocity with height that keeps cool, dry air entering from the upwind side while the warm, moist, rising air stays on the downwind side of the cell.

Suggestion: Discuss with the class how dangerous wind shear can be to aircraft. Also refer to microbursts below.

Supercell Thunderstorms—massive thunderstorms with a single circulation cell comprising very strong updrafts and downdrafts.

4.7.4 Microbursts. Microbursts are intense downdrafts that can accompany many severe thunderstorms. Their intensity can be sufficient to cause planes to lose air speed and consequent loss of lift, resulting in a crash.

4.7.5 Mesoscale Convective Systems. Large, organized masses of severe and supercell thunderstorms sometimes occur under unusual conditions. These are called mesoscale convective systems. In one situation, upper-air wind-flow patterns cause air to flow into a region and rise. The rising motion stimulates long-lasting clusters of severe and slowly moving thunderstorms. In another situation, the change in wind direction with height caused by the approach of a cold front makes air rise along a line ahead of the front. The resulting squall line of thunderstorms includes storms in different stages of development.

Derecho—a violent, straight-line windstorm that precedes the squall line.

4.8 Tornadoes: These storms, characterized by a dark funnel cloud, originating from the base of a cumulonimbus cloud, have the highest wind speeds of any storm on Earth. Some estimates go as high as 225 mi/hr (100 m/sec); although, most tornadoes move at about half that speed. Even so, the damage can be enormous when the funnel contacts the ground.

Suggestion: Mention to the class that wind speeds are usually measured with instruments such as an anemometer. These instruments would be destroyed if a tornado struck. Consequently, observers have to use indirect methods, such as assessing the degree of damage at a site, employing the Fujita Intensity Scale and its enhanced version developed in 2007. The scale starts at F0 (least powerful, least destructive) and can go as high as F5 (most powerful, most destructive).

4.8.1 Tornado Characteristics. A tornado appears as a dark funnel cloud hanging from the base of a dense cumulonimbus cloud. At its lower end, the funnel may be 100 to 450 m (about 300 to 1500 ft) in diameter. The base of the funnel appears dark because of the density of condensing moisture, dust, and debris swept up by the wind. Wind speeds in a tornado exceed those known in any other storm. Estimates of wind speed run as high as 100 m/s (about 225 mph), although generally they are closer to 50 m/s (about 110 mph). As the tornado moves across the countryside, the funnel writhes and twists. Where it touches the ground, it can cause complete destruction of almost anything in its path. The center of a tornado is characterized by low pressure, which is typically 10 to 15 percent lower than the surrounding air pressure. The result is a very strong-pressure gradient force that generates high wind speeds as the air rushes into the low-pressure center of the tornado. Most tornadoes rotate in a counterclockwise direction, but a few rotate the opposite way.

4.8.2 Tornado Development. Tornadoes are usually associated with the presence of severe thunderstorms, which provide one of the key ingredients in the initial development of the tornado—namely, a very strong vertical circulation. The other key ingredient is the presence of significant change in wind speed and direction with height, termed wind shear. In regions where there is significant wind shear, spinning circulations aligned with the ground—horizontal vortices—can form. Strong convection can then lift portions of the vortex, which results in a vertical tower of slowly rotating air called a mesocyclone.

4.8.3 Tornado Destruction. The large majority of tornadoes are relatively weak, lasting only a few minutes and covering only a few hundred meters. On the other hand, large tornadoes, while making up only about 5% of all tornadoes, can last for hours and spread destruction over hundreds of kilometers. Devastation from these tornadoes is often complete within the narrow limits of their paths. Only the strongest buildings constructed of concrete and steel can withstand the extremely violent winds. The most commonly used measure of tornado intensity is the enhanced Fujita Intensity Scale, or the EF-scale. This scale ranks tornadoes from 0 to 5, weakest to strongest. It is based on the severity of damage found in the tornado’s wake by examining 28 types of structures, with up to 12 different “degree of damage” ratings for each.

This scale allows scientists to compare tornadoes that have passed through very different regions (for example, industrial and rural areas), enabling them to better estimate the wind speed within each.

Suggestion: Have students examine various recent tornadoes, either in their immediate geographic vicinity or ones having captured national attention. Discuss the Fujita rankings of the selected tornadoes.

4.9 Air Quality: Most people living in or near urban areas have experienced air pollution first-hand.

Perhaps you have felt your eyes sting or your throat tickle as you drive an urban freeway, or you’ve noticed black dust on window sills or window screens and realized that you are breathing in that dust as well.

4.9.1 Air Pollutants. Pollutants in the form of aerosols, gases, and particulates can be injected into the atmosphere by both natural and human activities. Suffice it to say that this problem is overwhelmingly caused by humans. Auto exhaust, fossil fuel combustion for power grids, industrial pollution, etc., wind up in the atmosphere and easily spread to distant areas.

4.9.2 Fallout and Washout. Pollutants, especially those that are large particulates, ascend to higher elevations by convection, but return to Earth as fallout. Smaller particles can return to Earth via precipitation, a process simply called washout. Thus the combined processes of fallout and washout act to cleanse the atmosphere of pollutants.

4.9.3 Smog and Haze. Smog, a combination of “smoke” and “fog,” is a collection of aerosols and gaseous pollutants. Three main toxic ingredients (nitrogen oxides, volatile organic compounds (VOCs), and ozone) are found in urban smog. The first two ingredients come from cars and trucks, whereas ozone is formed by a photochemical reaction in which nitrogen oxide molecules react with VOCs in the atmosphere. Ozone in the lower atmosphere is harmful to human lungs.

4.9.4 Inversion and Smog. Pollutant concentrations can increase to very high levels in the lower layers of the atmosphere whenever vertical mixing (convection) is blocked. The culprit in this case is an inversion, a situation in which the air temperature actually increases commensurate with increases in altitude. A lid is established in the air where uplift is blocked, resulting in heightened concentrations of pollutants closer to the ground. These inversions form traps for many poisonous industrial gases and automobile exhaust.

Suggestion 1: Two unfortunate examples should be brought to the attention of the class. The first occurred in October 1948 in Denora, Pennsylvania, where an inversion lid in a valley trapped poisonous industrial gases for five days and resulted in 20 deaths and more than 5,000 persons becoming sick. A similar situation developed in London, where severe smog resulted in the deaths of over 4,000 in 1952 and 106 in 1962. Men had to walk in front of the buses with flashlights to guide them through the streets. In these cases, only a change in the weather was able to disperse the lid.

Suggestion 2: The notorious smog of the Los Angeles Basin in southern California results from a high-level temperature inversion caused by a warm layer of descending air lying over a cooler layer of ocean air that is closer to the surface. Add in the presence of a mountain barrier to the east of Los Angeles and you have all the ingredients for smog development. Mention to the class that the bulk of the population of California (the state with the largest population) lives at or near the coast between San Diego and San Francisco. Atmospheric circulation problems are one important reason that the state has been in the forefront of air-pollution-control measures for several decades.

Select Audiovisual Aids: NOVA Deadliest Tornadoes DVD (NOVA6240), Public Broadcasting System, Arlington, VA Numerous relevant videos available from ScienceDaily http://www.sciencedaily.com/videos/earth_climate/ Select Reference Books: Barry, Roger G., and Richard J. Chorley. Atmosphere, Weather and Climate, 8th ed., New York, NY: Routlege, 2003.

Oliver, John E. (Ed.). Encyclopedia of World Climatology, Dordrecht, Netherlands: Springer, 2005.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 5: Winds and Global Circulation Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe air pressure and how we measure it.

2) Discuss local wind patterns.

3) Compare cyclones and anticyclones.

4) Identify major global wind patterns.

5) Discuss the movement of air at higher levels of the troposphere.

6) Describe the causes and effects of ocean circulation patterns.

This chapter deals with the global circulation of air. Topics include all major aspects of air movement, ranging from areas of the world where winds prevail from one direction more than others to the relationship between global circulation and ocean currents. It concludes by discussing the effects of jet streams in the atmosphere.

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The Coriolis Effect: Earth’s rotation strongly influences atmospheric circulation through the Coriolis effect, which results from Earth’s rotation. The Coriolis force deflects wind motion, producing circular or spiraling flow paths around cyclones (centers of low pressure andconvergence) and anticyclones (centers of high pressure and divergence).

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Monsoons: The monsoon circulation of Asia responds to a reversal of atmospheric pressure over the continent with the seasons. A winter monsoon flow of cool, dry air from the northeast alternates with a summer monsoon flow of warm, moist air from the southwest.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface to their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are organized for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Atmospheric Pressure from Nova’s online adventure to Everest. One of the most extreme changes in atmospheric pressure that someone can experience is to climb Mount Everest. Find out here about the impacts upon climbers and the environment. http://www.pbs.org/wgbh/nova/everest/exposure/pressure.html Forces and Winds—the Online Meteorology Guide. A general overview of the concepts associated with the Coriolis force and winds. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/home.rxml Names of Winds. Discover the origins of the names of winds such as local and mountain winds. http://ggweather.com/winds.html

Bad Coriolis—cautionary website identifying mistakes people often make when describing or explaining the Coriolis effect. http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html Nova—Tracking El Nino. Find out how predictions of El Nino are made and what potential changes in the weather result from El Nino. http://www.pbs.org/wgbh/nova/elnino/ Year of the Ocean 1998. This NOAA online exhibition provides a rich overview of many aspects of Earth’s oceans and some of the problems they face. http://www.yoto98.noaa.gov/ Jet Stream Analysis from the California Regional Weather Server. http://squall.sfsu.edu/crws/jetstream.html Chapter Lecture: 5.1 Atmospheric Pressure: We live at the bottom of a vast volume of air that extends from Earth’s surface to a level that must be defined arbitrarily because there is no real boundary between the atmosphere and outer space. The vast bulk of the atmosphere is found closer to Earth’s surface. The pressure falls off rapidly as one ascends in elevation. Indeed, about 50% of the atmosphere lies below a height of 3.5 miles (5.63 kilometers). Atmospheric pressure is produced by the weight of the air above you. This weight at sea level is 14.7 pounds/in2 (1 kg/cm2).

5.1.1 Measuring Air Pressure. Air pressure is measured with a barometer, which uses the same principle as drinking soda through a straw. When using a straw, you create a partial vacuum in your mouth by lowering your jaw and moving your tongue. The pressure of the atmosphere on the liquid in your glass then forces soda up through the straw. The oldest, simplest, and most accurate instrument for measuring atmospheric pressure, the mercury barometer, works in the same way.

5.1.2 Air Pressure and Altitude. Air pressure falls off very rapidly near Earth’s surface, but the rate of decrease becomes slower as altitude increases. It is very common to experience an air pressure change in a fast-moving elevator in a tall building, by driving quickly through a mountainous area, or simply by sitting in a plane as it ascends. All these situations result in less oxygen entering the lungs, which in certain circumstances can result in mountain sickness.

Suggestion 1: This is a good place to talk about mountain sickness, which sets in at about 10,000 ft (3,000 m). Mention to the class the problems that occur when tourists from areas close to sea level fly into Denver, immediately rent a car to take a quick trip to the Rockies, and then wonder why they are lying on the ground, quietly discharging their gourmet airplane lunch and definitely missing the wonderful scenery.

Suggestion 2: This is also a good opportunity to mention the problems that airplane companies such as Boeing are having in building a plane made of composite plastics that are strong enough to let air pressure be higher in the cabin than is currently possible with aluminum. The result would of course benefit the passengers.

5.2 Wind: Wind is defined as air moving horizontally over Earth’s surface. Air motions can also be vertical, but these are known by other terms, such as updrafts or downdrafts. Wind direction is identified by the direction from which the wind comes—a west wind blows from west to east, for example. Like all motion, wind movement is defined by its direction and velocity. The most common instrument for tracking wind direction is a simple vane with a tail fin that keeps it always pointing

into the wind. Most wind currents are horizontal.. Wind speed is measured by a device called an anemometer.

Suggestion: Ask the class if they have ever flown across the continent and why there is a difference in flight times between planes flying east-to-west and west-to-east. The prevailing winds in the midlatitudes across the U.S. generally flow from west to east and thereby increase or decrease the flight time, depending on which way you are heading.

5.2.1 Pressure Gradients. Unequal heating in the atmosphere facilitates the development of pressure gradients. Air flows from areas of high pressure to those of low pressure. The larger the pressure difference between the two regions, the stronger the wind.

Suggestion: Use an analogy between high and low pressure for air and high and low elevations for water. Air will flow from the high pressure area to the low pressure area, just as water would flow downslope from the hill to the valley.

5.2.2 Local Winds. Local winds are caused by effects that are local. For example, sea and land breezes occur because water and land have very different specific heat values. Indeed, the difference is fivefold, with land being much quicker to warm up and cool down. Thus, the beach heats up rather quickly during the day, facilitating the movement of relatively cooler air from the ocean to the beach dwellers being toasted by the sun. Conversely, the land cools off much more rapidly at night, allowing land breezes to move towards the water. Mountain and valley breezes are also local winds that develop from the flow of cool air moving down the mountain during the night in contrast to the warm air moving upwards from the valley during the day as the air expands and rises. Although there are vertical aspects of mountain and valley breezes, they are similar to coastal sea and land breezes. Local winds also include the Santa Ana, a dangerous hot, dry wind that comes from the interior desert region of Southern California that rapidly descends the coastal mountains en route to the Pacific coast. It is associated with brushfires that can cause considerable damage. The chinook is a warm, dry wind that occurs when air descends on the leeward side of a mountain, such as the Rockies. It is locally called a “snow-eater” because of its ability to evaporate and melt snow. Other local winds that are cold and dry include the mistral in the Rhone Valley in southern France that not only can damage the vineyards (a serious issue in France), but also can cause vacationers on the Riviera to be chilled.

Suggestion 1: Mention to those in the class who enjoy this topic that there are easily over 100 local winds in the world, ranging alphabetically from the abroholos in Brazil to the zonda in Argentina.

Suggestion 2: The explosive growth in fires with Santa Ana winds is also associated with the existence of local chaparral vegetation that is loaded with very flammable oils.

Suggestion 3: Many students are interested in weather extremes. For example, chinooks are responsible for some astonishing temperature changes within a very brief period of time, such as a reported temperature change of 45º F (25º C) in less than a half hour.

5.2.3 Wind Power. The power of the winds has been used to move sailing ships for more than 3000 years and windmills for over 2000 years. Wind power has become much more attractive in recent years as a clean, renewable source of energy. New windmills are much taller and larger than the picturesque Dutch type, and several countries, such as Denmark, Germany, and the United States, have been developing wind farms that have become much more efficient.

Suggestion: Mention to the class that, although wind farms have their fans (pun intended), some groups oppose them on the ground that they will alter the scenery in the area. Just look at the opposition that has developed over a plan to build a wind farm in the coastal waters off Nantucket Island in Massachusetts because some residents on the island would have an unattractive seascape to look at rather than whitecaps.

5.3 Cyclones and Anticyclones: 5.3.1 The Coriolis Effect. Due to Earth’s rotation, any moving object on Earth, such as a rocket, wind, or an ocean current, is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The amount of deflection varies from zero at the equator to a maximum at the poles.

5.3.2 The Frictional Force. Closer to Earth’s surface, another force also affects the speed and direction of wind. As wind in the lower troposphere moves over Earth’s surface, a frictional force opposes the motion of the wind. In general terms, a rougher surface, with mountains, trees, or buildings, has a greater frictional drag on wind than a smooth surface. The frictional force is greatest close to the surface and decreases with altitude. The frictional force always acts in the direction opposite to the air motion.

5.3.3 Cyclones and Anticyclones. Cyclones are low pressure systems in which winds converge as air spirals inward and upward. This upward movement results in adiabatic cooling and consequent condensation and precipitation. The circulation of air around a cyclone in the northern hemisphere is counterclockwise due to the Coriolis effect and friction with the surface. As you may expect, the high or anticyclone has air that is sinking and spiraling outward. The air that is descending warms up at the adiabatic rate so that condensation does not occur. The result is generally fair weather. The circulation of an anticyclone is clockwise in the northern hemisphere, opposite to the circulation direction of a cyclone.

5.4 Global Wind and Pressure Patterns: Because the equatorial and tropical regions are heated more intensely than the higher latitudes, a vast thermal circulation develops—the Hadley Cells. This circulation drives the northeast and southeast trade winds, the convergence and lifting of air at the Intertropical Convergence Zone (ITCZ), and the sinking and divergence of air in the subtropical high-pressure belts. The polar front, lying between about 30° and 60° latitude, marks the boundary between warm, moist tropical air and cool, dry polar air. At the South Pole, outward-spiraling winds create the Polar Easterlies. The most persistent features of the global pattern of atmospheric pressure are the subtropical high pressure belts, which are generated by the Hadley Cell circulation. They intensify and move poleward during their high-Sun season, affecting the climate of adjacent coasts and continents. The northern belt includes the Hawaiian high, to the west of North America, and the Azores high, to the east.

5.4.1 Subtropical High-Pressure Belts. As a starter, let us examine the pattern of wind circulation on an “ideal Earth” that does not have oceans, continents, or seasons. The axial tilt of Earth results in more solar radiation reaching the equatorial areas than other regions.

Consequently, air moves upward over the Equator, rising to high altitudes where it forms a surface low-pressure zone known as the equatorial trough. This circulation is part of the Hadley Cell that develops on both sides of the equator, descending at about 30º north and south latitude, and results in subtropical high-pressure belts.

Suggestion: For the science historians in the class, mention the 1735 contribution of George Hadley in developing a cellular model to help explain Earth’s atmospheric circulation. His initial model was that of a huge single cell of circulation ranging from the equator to the poles in each hemisphere.

Later on, others modified this one-cell model to three cells in each hemisphere, leaving the term “Hadley Cell” to refer to circulation within the equatorial latitudes.

The circulation of air in the Hadley Cells results in the development of large, semi-permanent, high-pressure cells that expand and contract with the seasons. Two of these cells in the northern hemisphere (the Hawaiian and Azores or Bermuda Highs) are obviously associated with their proximity to certain islands in the Pacific and Atlantic Oceans.

Suggestion: Let the class know that additional and interesting climatic aspects of these high-pressure cells are discussed in later chapters.

5.4.2 The ITCZ and the Monsoon Circulation. The major part of this discussion is that air is rising in the equatorial zone and then descending at the aforementioned latitude of 30º in both hemispheres. This descending air converges in a narrow zone known as the Intertropical Convergence Zone (ITCZ). The ITCZ shifts with the seasons. The latitudinal shift is substantial, ranging from 20º in January in the southern hemisphere to an astonishing 40º in the northern hemisphere in July.

Suggestion: This is a good time to discuss briefly the significance of the ITCZ latitudinal shifts. To start with, the ITZC is associated with large amounts of precipitation as it moves from hemisphere to hemisphere, north and south of the Equator. If, for a number of reasons, the ITCZ does not pass over an area for several years, the effects can be disastrous. The drought of 1968−1974 in the Sahelian zone south of the Sahara Desert resulted in the estimated deaths of 100,000 persons and 5 million cattle.

The term monsoon originated from an Arabic word associated with a seasonal change in wind direction. The prime example is, of course, the reversal of atmospheric pressure in Asia, the largest land mass on Earth. Very high pressure during the winter over northeastern Asia produces a flow of cool, dry air that trends in a southerly direction. This wind pattern changes during the summer as warm and humid air comes off the Indian and Pacific Oceans and moves inland. The amount of precipitation that results can be wonderful for crops but can also cause flooding, if the amounts are too large. For a variety of reasons, including the fact that the North American landmass is less than that of Asia, the central and eastern portions of North America experience a marked tendency during the summer to have warm, moist air from the Gulf of Mexico move northward. This airflow pattern changes during the winter as dry, cold air from the interior of Canada moves south and eastward into the U.S. 5.4.3 Wind and Pressure Features of Higher Latitudes. At the present period in geologic time, the northern and southern hemispheres are characterized by radical differences in the location of continents and oceans. Most of the land area in the world is concentrated in the northern hemisphere as compared to the extensive area occupied by oceans in the southern hemisphere.

Suggestion: Mention to the class that plate tectonics and continental drift will be covered in Chapter 11 in order to provide perspective on the movement of land masses over the immensity of geologic time.

The differing patterns of land and water between the northern and southern hemispheres result in highly varying seasonal development of high- and low-pressure centers. For example, the Siberian and Canadian highs in the winter are characterized by cold, dry air that develops over the continental land masses. In contrast, the Aleutian and Icelandic lows develop over the northern Pacific and Atlantic Oceans respectively and are associated with winter storms. With the notable exception of Antarctica, the southern hemisphere is characterized by large ocean areas. Given the areal extent of Antarctica and its high elevation, it is always cold and dry, enabling a permanent anticylone (the South Polar high) to exist year-round.

Suggestion: For the Antarctica buffs in the group, mention that in addition to its position over the South Pole, the ice cap rises to nearly 10,000 feet (3,000 m) and in one part of East Antarctica reaches elevations over 13,000 feet (4,000 m).

5.5 Winds Aloft: We’ve looked at airflows at or near Earth’s surface, including both local and global wind patterns. But how does air move at the higher levels of the troposphere? Like air near the surface, winds at upper levels of the atmosphere move in response to pressure gradients and are influenced by the Coriolis effect. How do pressure gradients arise at upper levels? A simple physical principle states that pressure decreases less rapidly with altitude in warmer air than in colder air. Also recall that the solar energy input is greatest near the Equator and least near the poles, resulting in a temperature gradient from the Equator to the poles. This gives rise to a global pressure gradient.

5.5.1 The Geostrophic Wind. Let us begin with the fact that winds near Earth’s surface have to respond to three influences: 1) the pressure gradient force (from high to low), 2) the Coriolis effect, which imparts a rightward direction to moving air in the northern hemisphere and a leftward direction in the southern hemisphere, and 3) surface friction. At upper levels the surface friction factor is not applicable; therefore, the air moves parallel to the isobars as the pressure gradient and Coriolis forces balance out.

Suggestion: For review, refer to earlier diagrams in the text concerning wind flow and how it crosses the isobars at acute angles.

5.5.2 Global Circulation at Upper Levels. The global, upper-level winds include: 1) westerly winds that spiral around a large, polar, low-pressure center, 2) upper-air westerlies in the midand high latitudes, 3) a tropical high-pressure belt at about 15º−20º N and S latitude, and 4) a zone of light equatorial easterlies.

5.5.3 Jet Streams and the Polar Front. The juxtaposition of strong atmospheric pressure gradients and high altitudes (30,000−40,000 ft [9−12 km]) results in jet streams that can reach 170−280 mph (75−125 m/s). This rapidly moving air in the upper parts of the troposphere is subdivided into three categories: 1) polar jets, 2) subtropical jets, and 3) tropical easterly jets.

The polar jet is usually found between 35º and 65º in both the northern and southern hemispheres. At these latitudes, the jet stream follows the boundary between the colder polar air and the warmer subtropical air.

Suggestion: For the military history buffs in your class, it is worthwhile to mention the impact of jet streams on the bombing runs carried out by the United States on Japanese-held islands in the Pacific during World War II. Many of the planes were starting to run out of fuel as they proceeded westward and had to jettison their bomb loads. They then noticed how quickly they got back to their bases that were located in an easterly direction. The obvious culprit was the jet stream. At present, commercial

flights between New York and Europe are an hour faster going (outbound eastward) that coming (returning westward).

5.5.4 Disturbances in the Jet Stream. Jetstream disturbances are broad, wave-like undulations found at high altitudes in the midlatitudes. They are also called Rossby Waves in honor of meteorologist C.G.A. Rossby, who first observed and studied them. These waves are a very important part of heat transfer in the westerlies, inasmuch as they facilitate the movement of cold air to equatorial regions and warm air to polar regions. This mitigates the unequal distribution of heat and cold on the planet that is due to variations in latitudinal inputs of solaradiation.

Suggestion: This is an excellent time to mention the importance of atmospheric circulation in the movement of warm and cold air that makes Earth much more habitable. Note that the circulation of the next fluid discussed, marine water, also plays a role in atmospheric circulation and global temperature.

5.6 Ocean Circulation: Just as there is a circulation pattern to the atmosphere, there is also a circulation pattern to the oceans that it is driven by differences in density and pressure acting in concert with the Coriolis effect. Pressure differences are created in the water when the ocean is heated unequally, because warm water is less dense than cold water. These pressure differences induce the water to flow. Similarly, saltier water is denser than water that is less saline; therefore, differences in salinity can also cause pressure differences. The force of wind across the water’s surface also creates oceanic circulation.

5.6.1 Temperature Layers of the Ocean. The oceans are characterized by a simple layer of thermal zones, with upper layers near the surface having the highest temperature and (as you might expect) water at depth being the coldest. A warm surface layer develops at low latitudes throughout the year and during the high-sun period in both hemispheres in the middle latitudes. The warm surface layer can be as thick as 1,640 ft (500 m) with a temperature of about 68º−77º F (20º−25º C) in waters close to the Equator. The next layer is called the thermocline; it separates the warm upper layer from the cold water that descends to the ocean floor. Here temperatures can range from 32º to 41º F (0º to 5º C).

Suggestion: For those who like to swim in lakes, mention that thermoclines also exist in these freshwater bodies. The depths are, of course, much shallower than those in the oceans. Depending upon the size of the lake, the thermocline can be relatively close to the surface, so anyone who dives in can experience the chill in the deeper layers.

5.6.2 Surface Currents. In a manner analogous to warm air moving poleward and cold air moving equatorward, warm sea currents that move poleward and cold sea currents that move equatorward act to maintain the global energy balance. On a global scale, two very large, wind-driven, circular gyres are found in both hemispheres at latitudes of 20º−30º. In the Atlantic, the dominant warm currents are the Gulf Stream and the North Atlantic Drift, and the major cold current is the Labrador Current.

Warm Current Suggestion: Mention to the class the significance of the North Atlantic Drift in World War II, as this warm current is powerful enough to reach the northern parts of Scandinavia and keep the port of Murmansk ice-free all year so that much-needed military supplies from the U.S. and Britain could be successfully delivered to Russia, even though its location at latitude 69º N was well above the Arctic Circle.

Cold Current Suggestion: Two interesting aspects of the Labrador Current that flows equatorward between Labrador and Greenland, and continues on to reach the coasts of Newfoundland, Nova Scotia, and New England, are worthy of mention. Portions of the Greenland Ice Cap calve (break off) and drift south as icebergs. On the night of April 14−15, 1912, one of these icebergs collided with a vessel that was thought to be the fastest ship afloat and also unsinkable. The remains of the Titanic now rest on the ocean floor as a monument to the 1500 that drowned. On a more pleasant and adventurous note, anyone wanting to test the waters touched by the Labrador Current is encouraged to swim along the coast of Maine during the heat of summer. It may be one of the quickest dips in the water you will ever have.

5.6.3 El Nino and ENSO. Another aspect of surficial ocean currents occurs in the Pacific and is called El Nino events. These events are related to changes in barometric pressure along the equatorial part of the Pacific and have been called the Southern Oscillation, or ENSO. The usual upwelling along the Peruvian coast stops and global patterns of precipitation change.

The Humboldt Current that usually flows along the western coast of South America is replaced by warm water from the west, which drastically affects the fishing industry in this region.

5.6.4 Pacific Decadal Oscillation. Changes in sea-surface temperatures in the northern Pacific can affect the climate in portions of Eurasia, Alaska, and the western U.S. ENSO-related changes occur in cycles of 20−30 years.

5.6.5 North Atlantic Oscillation (NAO). The NAO is connected with atmospheric pressure and sea-surface temperature changes in the middle and high latitudes in the Atlantic. The NAO has two phases: 1) a positive phase that keeps the polar jet to the north, resulting in warmer weather to the southeast, while northern Europe experiences an increase in stormy weather, and 2) a negative phase where the polar jet swings further south, resulting in colder conditions in the eastern U.S. and an increase in precipitation in the Mediterranean area. The time scale of changes in the NAO varies from weeks to decades.

5.6.6 Deep Currents and Thermohaline Circulation. Deep ocean currents that are generated by cold, salty water sinking in the northern Atlantic act to circulate sea water in a series of slowly moving loops that result in upwelling in the Pacific and Indian oceans. This pattern of thermohaline circulation affects most of the ocean basins in the world and, more importantly, acts as a constraint on the increase of atmospheric carbon dioxide by transporting surface waters that are rich in CO2 to deeper levels of the oceans.

Select Audiovisual Aids: NOVA Tracking El Niño DVD, Public Broadcasting System, Arlington, VA Numerous relevant videos available from ScienceDaily http://www.sciencedaily.com/videos/earth_climate/ Select Reference Books: Barry, Roger G., and Richard J. Chorley. Atmosphere, Weather and Climate, 8th ed., New York, NY: Routlege, 2003.

Oliver, John E. (Ed.). Encyclopedia of World Climatology, Dordrecht, The Netherlands: Springer, 2005.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 6: Weather Systems Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe air masses and how they interact to form fronts.

2) Contrast the characteristics of midlatitude cyclones and anticyclones.

3) Describe how tropical cyclones develop, where they go, and identify their impacts.

4) Identify the mechanisms of poleward energy transport.

There is a lot of information to cover in this chapter with a few topics that particularly interest students, such as tornadoes and hurricanes.

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Air masses and mid‐latitude cyclones: Compared to severe weather such as tornadoes and hurricanes, students may feel that the relatively sedate mid‐latitude cyclones are not as interesting or important.

Alternatively, use a recent example of a particular cyclone that may be memorable, such as a storm that produced heavy rain.

Another technique is to use a historic example of weather such as the weather patterns that delayed the D‐Day landings in World War II. Tornadoes: There are many aspects of tornadoes that students find interesting, but they may not appreciate that we still have much to learn about them. In addition, thanks to disaster movies and a burgeoning tourism industry, students may not appreciate that tornadoes are extremely hazardous.

Direct students to the Valparaiso University site to find out how researchers organize the monitoring of a tornado. A very useful site for those occasional students who express a desire to chase tornadoes!

The Fujita Scale site is a useful way to demonstrate that tornadoes vary considerably and as result so does the damage they create.

Hurricanes: Students also find hurricanes compelling, but again may not fully understand the processes that produce them.

Depending on the time of year you are teaching this topic, you may be able to describe a hurricane as it develops and travels across the Atlantic. Use the first five minutes of a sequence of classes to monitor a storm, and explain the processes involved. The National Weather Service site provides this near‐real‐time information.

The Saffir‐Simpson Scale site is a useful way to demonstrate that hurricanes also vary considerably and as result so does the damage they create.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are organized for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Air Masses and Fronts ‐Online Meteorology Guide. A brief introduction to the processes involved with air masses and fronts. http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/af/home.rxml Reading Weather Maps ‐Online Meteorology Guide. A useful explanation of weather maps and how to use them. http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/home.rxml Tropical Rainfall Measuring Mission (TRMM). This current research project is described in the text.

Go to this site for current information on the project. http://trmm.gsfc.nasa.gov Hurricanes—online meteorology guide. A general review on topics relating to hurricanes. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hurr/home.rxml National Weather Service Tropical Prediction Center. Find out how tropical storms are predicted and tracked. http://www.nhc.noaa.gov/index.shtml The Hurricane Hunters—The 53rd Weather Reconnaissance Squadron flies through hurricanes to take direct measurements. They even post their photographs on this website as soon as they land. http://www.hurricanehunters.com/ Saffir‐Simpson Hurricane Scale describes the conditions associated with categories of hurricanes and some examples of past hurricanes. http://www.aoml.noaa.gov/general/lib/laescae.html Hurricane Information from USA Today. A general guide to hurricanes with some safety information. http://www.usatoday.com/weather/hurricane/whur0.htm Tornado Project Online. This extensive site provides a wide array of information on tornadoes from researchers and professional storm chasers. http://www.tornadoproject.com/index.html Valparaiso University Storm Intercept Team. A site about one research team that chases tornadoes. http://www.valpo.edu/student/vusit/ Edmonton Tornado, 1987—devastating event in the history of this Canadian city. http://datalib.library.ualberta.ca/tornado/ Tornado Information from USA Today. A general guide to tornadoes with some safety information. http://www.usatoday.com/weather/tornado/wtwist0.htm

Chapter Lecture: 6.1 Air Masses and Fronts: Air masses are large bodies of the atmosphere that are characterized by fairly homogeneous physical properties, particularly temperature and moisture. They develop over land and water source areas and can move from place to place by pressure gradients and wind patterns at upper levels of the atmosphere. As they move from their source areas, their properties can be modified as they travel over land or water. The air masses that affect North America include: 1) the cold and dry continental and arctic (cP and cA) from northern Canada and Alaska, 2) the cool and moist maritime polar (mP) from the northern Pacific and Atlantic oceans, 3) the warm and moist maritime tropical (mT) from the Gulf of Mexico, portions of the Atlantic near the eastern seaboard, and an area in the Pacific southwest of Mexico, and 4) a hot, dry continental tropical (cT) that develops in the summer in the southwestern U.S. and northern Mexico.

Suggestion: Most students are intrigued by weather extremes. For example, an unusually cold and dry air flow (the continental arctic [cA]) that started near the Arctic Circle continued moving in a southeasterly direction and within several days reached north‐central Florida. By Dec. 24, 1989, Tallahassee and Orlando (near Disneyland) at 30.5º and 28.5º north latitude had low temperatures of 14º F (−10º C) and 22º F (−5.6ºC), respectively.

6.1.1 Types of Fronts. A given air mass usually has a sharply defined boundary between itself and a neighboring air mass. This boundary is termed a front. A cold front occurs when a cold-air mass temporarily invades a zone occupied by a warm air mass during the passage of a weather system. Because the cold air mass is colder and therefore denser than the warmer air mass, it remains in contact with the ground. As it moves forward, it forces the warmer air mass to rise above it. As the warm air mass rises, it cools adiabatically, water vapor condenses, and clouds form. If the warm, moist air is unstable, severe convection may develop. A cold front often forms a long line of massive cumulus clouds stretching for tens of kilometers (or miles). A warm front is one in which warm air moves into a region of colder air, as the cold air retreats. Again, the cold air mass remains in contact with the ground because it is denser. As before, the warm-air mass is forced aloft, but this time it rises up on a long ramp over the cold air below. This rising motion, called overrunning, creates stratus—large, dense, blanket-like clouds—that often produce precipitation ahead of the warm front. If the warm air is stable, the precipitation will be steady. If the warm air is unstable, convection cells can develop, producing cumulonimbus clouds that provide heavy showers or thunderstorms.

Suggestion: The development of frontal movement models owes a great deal to a group of Norwegian scientists during World War I. At that time, the Norwegians were unable to obtain weather reports from the Atlantic and other places. They quickly established a fairly dense network of weather stations in their country and developed very useful advances in atmospheric modeling that facilitated an understanding of frontal movement that is still being used today.

6.2 Midlatitude Anticyclones and Cyclones: When the Coriolis force, the pressure gradient force, and friction interact, air spirals inward and converges in a cyclone, while air spirals outward and diverges in an anticyclone. Most types of cyclones and anticyclones are large features that move slowly across Earth’s surface, bringing changes in the weather as they move. These are referred to as traveling cyclones and anticyclones.

6.2.1 Anticyclones. Anticyclones are characterized by dry, clear air that moves eastward and equatorward from its source area in higher latitudes. Adiabatic heating occurs as the traveling anticyclone descends and diverges, resulting in typically fair weather conditions.

The barometric pressure is high at the center of an anticyclone.

6.2.2 Cyclones. Cyclones have air that is converging and cooling adiabatically as it rises.

When the air reaches the saturation point, condensation and precipitation usually occur.

Midlatitude cyclones are the major weather system in the middle and high latitudes. They develop, strengthen, and eventually dissolve along the polar front ‐the boundary between two major anticyclones: the cold and dry polar high to the north and the warm, moist subtropical high to the south. The midlatitude cyclones usually move eastward due to the prevailing westerly winds in the upper air.

Suggestion: Try this out on your class. On the assumption that most students have used a nutcracker, you could illustrate the various stages in the development of a wave cyclone that eventually develops into an occluded front as if the two handles of a nutcracker were moving closer and closer together to force the warmer air to rise well above the ground and precipitate (or break the nuts if you were dining).

6.2.3 Midlatitude Cyclones. The midlatitude cyclone is the dominant weather system in middle and high latitudes. It is a large in-spiraling of air that repeatedly forms, intensifies, and dissolves along the polar front. Midlatitude cyclones develop along the polar front which sits between two large anticyclones: the polar high to the north, with its cold, dry air mass, and the subtropical high, with its warm, moist air mass, to the south. At the polar front, the airflow converges from opposite directions, with northeasterly winds to the north of the polar front and southwesterly winds to the south of the polar front. These wind motions lead to a counterclockwise, or cyclonic, circulation that creates a low-pressure trough between the two high-pressure cells. Midlatitude cyclones are local intensifications of cyclonic circulation that move along this low-pressure trough. The circular motion of air around the cyclone generates warm and cold fronts that sweep across large regions, prompting weather changes.

6.2.4 Midlatitude Cyclones and Upper‐Air Disturbances. These often grow over time as a consequence of jet-stream disturbances that are responsible for creating areas of upper‐air convergence and divergence. This situation results in surficial circulation that travels eastward with the disturbances.

6.2.5 Cyclone Tracks and Cyclone Families. Cyclone tracks and cyclone families develop as upper‐air and surface pressure patterns get linked by vertical circulation related to convergence and divergence. Three examples of this fairly common situation are: 1) frequent jet-stream disturbances associated with the semi-permanent Aleutian and Icelandic Lows in the northern Pacific and Atlantic oceans, 2) mountain chains such as the Rockies that produce lee‐side troughs that can lead to cyclones, and 3) areas that experience strong land‐sea thermal contrast that develops into disturbances and cyclones. Note the common storm tracks that are associated with midlatitude cyclones that form in areas such as the Aleutian and Icelandic low‐pressure zones. These systems move along with the prevailing westerlies and often result in cyclone families that go through the various phases of their life, from an early stage to occlusion to a dissolving stage, as they move northeastward along a storm track.

6.2.6 Cold‐Air Outbreaks. Cold‐air outbreaks occur when cold polar air from the midlatitudes moves into very low latitudes, especially in North and South America. If the outbreak is severe enough, subfreezing temperatures can damage subtropical and tropical crops such as citrus and coffee.

6.3 Tropical and Equatorial Weather Systems: As might be expected, there are some differences between midlatitude and polar weather systems and those associated with the tropics and equatorial zones. In the low latitudes, air‐mass movement is slower and more gradual, and weakness is a characteristic of upper‐air winds. In addition, fronts are not as clearly defined as in the midlatitudes, as the air masses are generally warm and moist. A common form of a tropical weather system in the 5º−30º N and S latitudinal belt is the easterly wave, a trough of low pressure within the trade wind zone but not over the equator itself. The weak equatorial low is a related weather system with plentiful rain that generally forms along the ITCZ. 6.3.1 Tropical Cyclones. Tropical Cyclones rank at or near the top in terms of destructive power, wind speed (65‐135 mph; 30‐50 m/s), and unusually low barometric pressure (28.1 in. Hg; 950 mb). These storms are called hurricanes in the western hemisphere, typhoons in the western Pacific, and cyclones in the Indian Ocean. They develop over oceans between 10º and 20º N and S latitudes but never closer than 5º to the Equator. Easterly waves or weak lows that can intensify are the usual originators of tropical cyclones. Following their formation, these storms can intensify in both wind speed and precipitation as they move westward through the trade‐wind belt and then enter the midlatitudes where their trajectories become influenced by the prevailing westerlies. Based on the enormous spiraling of these storms, an eye can develop in its center that is notable for its clear skies and calm winds.

The very rapid spinning of the storm and the adiabatic warming of air as it comes down from high altitudes creates this unusual moment of peace (about a half hour).

Suggestion: Most students in your class are already aware of the power of tropical cyclones, either by direct experience or through the media. Add another item to their list. A major U.S. naval task force was caught in 1944 by a severe typhoon off the Philippines. The damage was simply unbelievable. Some smaller ships were sunk, and the flight deck of a large carrier was ripped apart so badly that it was out of commission for the remainder of the war.

6.3.2 Tropical Cyclone Development. Tropical cyclone development and its associated intensity are based on the positive feedback between the atmosphere and the ocean that facilitates the transfer of enormous amounts of energy from the ocean to the atmosphere.

The convective lifting of air, as an easterly wave passes over the warm ocean waters, leads to condensation, which in turn releases latent heat, which in turn results in more convection, and so on. In a sense, the system is spinning out of control. Add in the relative weakness of the Coriolis force at the low latitudes, where tropical cyclones develop, and you have another factor that allows wind speeds to reach higher values than in traveling cyclones in the midlatitudes.

Suggestion: The above comment about positive feedback loops is a good example of an unstable system that moves farther and farther away from its initial values. The opposite would occur in a negative feedback loop in which the system would return to its initial values, thereby maintaining homeostasis.

6.3.3 Tropical Cyclone Tracks. Tropical cyclones occur only during certain seasons. For hurricanes of the North Atlantic, the season typically runs from June through November, with maximum frequency in late summer or early autumn. In the southern hemisphere, the season is roughly the opposite. These periods follow the annual migrations of the ITCZ to the north and south, with the seasons, and correspond to periods when ocean temperatures are warmest. Most of the storms originate at 10° to 20° N and S latitude and tend to follow known tropical cyclone tracks. In the northern hemisphere, they most often travel westward and northwestward through the trade winds, and then turn northeast at about 30° to 35° N latitude into the zone of the westerlies. Here their intensity lessens, especially if they move over land. In the trade-wind belt, the cyclones typically travel at 10 to 20 km (6 to 12 mi) per hour. In the zone of the westerlies, their speed is more variable.

6.3.4 Impacts of Tropical Cyclones. Tropical cyclones can be enormously destructive due to the combined effects of intense winds and copious rainfall. In order to be classified as a hurricane, tropical cyclones must attain sustained winds of at least 74 mph (33 m/s). The Saffir‐Simpson scale ranges from category 1 (minimal damage; winds of 74‐95 mph; 33‐42 m/s) to category 5 (catastrophic damage; winds greater than 155 mph; 70 m/s).

Suggestion: Advise the class that the Saffir‐Simpson scale for hurricanes and the Richter scale for earthquakes have a common feature, namely that the damage does not increase linearly. For example, a linear increase for the Saffir‐Simpson scale would show relative damage for the five categories increasing at the linear rate of 1, 2, 3, 4, and 5. However, the relative damage increases at the rate of 1, 4, 40, 120, and 240. The numbers are simple, but the difference is substantial.

Tropical cyclones are also very capable of inflicting severe damage to coastal locations by storm surges- such as Hurricane Sandy. These rapid rises in water level are due to four factors: 1) the very low atmospheric pressure at the center of the storm allows the water level to rise higher than normal, 2) strong winds can result in damaging surf that can add to the rise in sea level, 3) a high tide that occurs at the same time as the storm can obviously raise the inundation to even higher levels, and 4) the underwater shape of the bay floor can also add its “two cents” to higher water levels.

Suggestion: As if the above four factors were not enough, just remind the class about rising sea level and the increase in population along the coast. The disaster scenarios would delight some Hollywood producers.

6.3.5 Impacts on Coastal Communities. Impacts on coastal communities by storms can be, regrettably, very substantial. Accordingly, the number of examples is, again regrettably, quite large. Brevity dictates two examples. Galveston Example: As a starter, you could talk about the unnamed hurricane that hit Galveston, Texas on Sept. 8, 1900. This city on a barrier island in the Gulf of Mexico is about 10 ft (3 m) above sea level. The estimated wind speed at landfall was about 135 mph (215 km/h), which would have made it a category 4 storm in the Saffir‐Simpson scale. The estimated storm surge was 16 ft (5 m), resulting in an estimated death toll of 6,000‐8,000. The city later built a 10-mile (16-km) long protective seawall to fend off the storms, but other hurricanes have occurred with considerable damage, such as Hurricane Ike on Sept. 13, 2008 with winds of 110 mph (177 km/hr) and a storm surge of 13.5 ft (4.1 m). Bangladesh Example: This country with an area of only 55,598 sq mi (143,998 sq km) has a population of over 155 million people, making it one of the most densely populated nations in the world. Most of this population lives on the floodplains and

deltas of the Ganges and Brahmaputra Rivers. Over 30% of the land surface, especially in the delta region, is less than 19.7 ft (6 m) above sea level. The 1970 cyclone and its associated storm surge reached 23 ft (7 m), resulting in an estimated death toll of 400,000 people. Another cyclone in 1990 drowned 148,000 people, with a storm surge of 19.7 ft (6 m). To make matters worse, Bangladesh is located at the apex of the Bay of Bengal, so that the tidal effects, when they occur, are accentuated. Enough said.

6.4 Poleward Transport of Heat and Moisture: As alluded to in previous comments, Earth has two major avenues for heat distribution from the equatorial and tropical areas to the much colder polar zones: the atmosphere and the oceans. The Hadley Cell circulation and the movements of warm and cold‐air masses in jet stream disturbances in the atmosphere are wondrous mechanisms for balancing worldwide thermal contrasts. Oceanic global circulation results in warm surface water from the equatorial and tropical areas, moving poleward via the Atlantic Ocean. As it reaches the North Atlantic, the surface water sinks to the bottom. Europe is the main beneficiary of this example of thermohaline circulation, as warm air is able to move eastward due to the prevailing westerlies.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (1‐800‐255‐8629): NOVA: Wild Weather, set of 3 programs, 60 minutes each, VC‐3500V; DV‐3571.

Crystal Productions, Glenview, IL: What Makes Weather, 14 minutes, VC‐3315.

Films for the Humanities and Sciences (800‐257‐5126): Atmosphere, Climate and Weather, DVD OUR347‐26‐K; VHS OUR‐347‐26‐A. Insight Media, New York, NY (800‐233‐9910): Weather and Climate, 15 minutes, DVD 79‐AS‐1266, 2000.

Insight Media, New York, NY: Understanding Weather Concepts, 23 minutes, DVD # 79‐AS‐1217, 2002.

Select Reference Books: Hewitt, C. Nick and Andrea V. Jackson. Atmospheric Science for Environmental Scientists. Wiley, 2009.

Martin, Jonathan E. Mid‐Latitude Atmospheric Resources: A First Course. Wiley, 2006.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 7: Global Climates Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Identify factors that differentiate the world’s climates 2) Explain how climates are classified.

3) Identify low-latitude climates.

4) Identify midlatitude climates.

5) Identify high-latitude climates.

The systematic and sequential nature of global climates and climate classification can make it difficult to avoid teaching and learning by rote. There is also a lot of information and data that can seem overwhelming to students. Here are a few suggestions on how to get this information across.

• Spread it out—Rather than teaching all the climates as one block, introduce one or two of the thirteen climate types throughout the course, perhaps with relevant soils and vegetation.

• Compare and contrast—Teach two or three climate types together and compare them to one another. For example, group wet tropical and dry tropical.

• Explore climates using the web sites listed below—Assign students to assess the climate of a particular place using the World Climate site. Challenge them to identify which features of the climate type are evident in the climate of their chosen place.

• Point students to the Live Weather Images site and challenge them to describe and explain the weather. Alternatively, access a different weather web cam at the beginning of each class and discuss its weather characteristics.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface to their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

World Climate. Type in the location to find out about its climate. http://www.worldclimate.com/ Climates of the World. A forty page guide from the National Oceanic and Atmospheric Administration. http://www.ncdc.noaa.gov/oa/documentlibrary/pdf/climatesoftheworld.pdf

Maps for Climate Classification (FAO). This FAO site has a particular focus on the impact of climates on agricultural production. http://www.fao.org/WAICENT/faoinfo/sustdev/EIdirect/climate/EIsptext.htm National Climatic Data Center. The NCDC collects and analyzes global climate data and many meteorological variables. http://lwf.ncdc.noaa.gov/oa/ncdc.html Interactive Koppen’s Climate Classification Map. Click on the locations on the map to view their climographs. http://www.uwmc.uwc.edu/geography/100/koppen_web/koppen_map.htm Global Extremes. A NOAA site that records and describes the extremes of weather around the world, such as extreme temperatures. http://lwf.ncdc.noaa.gov/oa/climate/globalextremes.html Live Weather Images—Weather Cams. Access live pictures from around the world to see their current weather conditions. http://www.weatherimages.org/weathercams/ The Tropical Meteorology Project. A research project that aims to evaluate tropical weather phenomena such as typhoons. http://typhoon.atmos.colostate.edu/ PBS Nature—Monsoon. An in‐depth and dynamic site from PBS describing the processes related to monsoons and why monsoons are variable. http://www.pbs.org/wnet/nature/monsoon/html/intro.html The Sahara Desert. A site describing the physical geography, biology, and culture of the Sahara. http://www.oxfam.org.uk/coolplanet/ontheline/explore/nature/deserts/sahara.htm UN Convention to Combat Desertification. The results of international cooperation to reduce desertification by protecting natural vegetation and monitoring climates. http://www.unccd.int/en/programmes/Pages/home.aspx Tundra Page. A useful review of tundra climates and the biomes of that region. http://ths.sps.lane.edu/biomes/tundra3/tundra3.html Taiga Net. A general description of the vegetation of the taiga and some of the environmental concerns today. http://www.taiga.net/index.html Arctic Climate and Meteorology Primer (from National Snow and Ice Data Center). http://nsidc.org/arcticmet/index.html Boreal Ecosystem Atmosphere Study (BOREAS). A research project examining many aspects of boreal climates, including greenhouse gases. http://daac.ornl.gov/BOREAS/boreas.shtml The American Experience—Surviving the Dust Bowl. A historical account of the Dust Bowl of the 1930 when thousands had to abandon their farms because of soil erosion and drought. http://www.pbs.org/wgbh/amex/dustbowl/

El Nino and California Precipitation. A thorough explanation of how El Nino affects rainfall in California. http://tornado.sfsu.edu/geosciences/elnino/elnino.html Managing Arid and Semi‐Arid Watersheds. A detailed and practical introduction on managing arid regions to prevent desertification. http://ag.arizona.edu/OALS/watershed/index.html

Chapter Lecture: 7.1 Keys to Climate: Climate is simply the average weather of a selected area of Earth. Average temperature and precipitation in conjunction with their seasonality characterize these different areas. Major climatic factors include latitude, elevation, coastal proximity, and the presence and duration of major air masses.

7.1.1 Global Monthly Temperature Patterns. The month-to-month variation in temperature at a location depends on latitude, location, and elevation. At higher latitudes, in continental interiors, and at higher elevations, the variation is greater.

Temperatures are cooler with elevation because the atmosphere cools at the average environmental temperature lapse rate of 6.4°C/1000 m (3.5°F/1000 ft).

7.1.2 Global Precipitation Patterns. Global Precipitation Patterns reflect aspects of world‐wide atmospheric circulation that in turn dictate the movement of air masses. Note that the following set of seven precipitation regimes also reflect the location of high‐ and low‐pressure zones that are in accord with the circulation systems found on Earth. The seven global precipitation regimes are: 1. Wet equatorial belt. Moist mE air masses converge in this equatorial zone of warm temperatures to generate abundant convective rainfall; 2. Trade-wind coasts. Here, low-latitude easterly trade winds move moist mT air masses from warm oceans to land, where hills and mountains generate orographic rainfall; 3. Tropical deserts. Stationary subtropical high-pressure cells, driven by Hadley cell circulation, provide hot and dry subsiding cT air masses that produce barren deserts; 4.

Midlatitude deserts and steppes. In midlatitude continental interiors, far from oceanic moisture sources, dry air masses dominate, and precipitation is low; 5. Moist subtropical regions. These midlatitude regions lie on the western sides of subtropical high-pressure cells, where they receive moist mT air masses from tropical oceans that provide ample cyclonic and convective precipitation; 6. Midlatitude west coasts. Prevailing westerly winds bring cool and moist mP air masses to mountainous west coasts, producing abundant orographic precipitation; 7. Arctic and polar deserts. These regions are dominated by cold cP and cA air masses that are too cold to hold much moisture, and precipitation is low.

7.1.3 Global Monthly Precipitation Patterns. Global monthly patterns of precipitation are typically of three types: 1) Uniformly distributed precipitation. This includes a wide range of possibilities, from little or no precipitation in any month to abundant precipitation in all months; 2) Precipitation maximum during the summer (or season of high Sun). Plants grow best during the season with highest insolation, and if the warm season is also wet, growth of both native plants and crops is enhanced; and 3)

Precipitation maximum during the winter or cooler season (season of low Sun). This means that the warm season is also dry. The stress on growing plants can be intense, and crops will most likely require irrigation.

7.2 Climate Classification: Climate Classification is best based on air‐mass movements and frontal zones. Two of the most important variables in climate (temperature and precipitation) are major attributes of an air mass. Frontal zones form if the air masses differ. Accordingly, the annual cycles of temperature and precipitation are governed by the seasonal movements of air masses and frontal zones.

7.2.1 Climate Classification Systems. Geographers and applied climatologists have devised a number of classification systems to group the climates of individual locations into distinctive climate types. Like all other scientific models, these classification systems are not exact replicas of the real-world climates they represent, but they do provide a useful way to summarize, communicate, and exchange information about them.

One climate classification system still in wide use today was developed by the Austrian climatologist Vladimir Köppen in 1918 and modified by Rudolf Geiger and Wolfgang Pohl in 1953. Primarily designed to capture variability of vegetation around the globe, it features a system of letters to label and define climates by mean annual precipitation and temperature, as well as precipitation in the driest month. The Köppen climate system is easy and convenient to use, given monthly weather data, but it is not directly related to the underlying processes that differentiate Earth’s climates.

Suggestion: This is a good place to discuss the differences between the two climate systems mentioned in this text, as the Köppen system is entirely empirical, i.e., based solely on the observation of temperature and precipitation. The resulting boundaries are integrated with known vegetation and soil boundaries. This system has some advantages, namely that only precipitation and temperature means are needed; therefore, it is simple to apply. The disadvantage is that it does not deal with causes. In contrast, the Strahler classification based on air-mass source region and frontal zones is explanatory, descriptive, and genetic because it accounts for both cause and effect. It is generally recognized that the best system of natural science classification is genetic.

7.2.2 Climate Groups. By combining what we know about air-mass source regions and frontal zones, we can subdivide the globe into bands according to latitude in order to define three broad groups of climates: low-latitude (Group I), midlatitude (Group II), and high-latitude (Group III). Within each of these three climate groups are several climate types (or, simply, climates)—four low-latitude climates, six midlatitude climates, and three high-latitude climates—for a total of 13 climate types.

7.2.3 Dry, Moist, and Wet-Dry Climates. All but two of our 13 climate types are classified as either dry climates or moist climates. In dry climates, evaporation and transpiration exceed precipitation by a wide margin. In moist climates, precipitation maintains soil moisture and active stream flow for most or all of the year. Wet-dry climates alternate between wet and dry states.

7.2.4 Highland Climates. Highland climates occupy mountains and high plateaus at any latitude. They tend to be cool to cold because air temperatures in the atmosphere normally decrease with altitude. They are also often moist from orographic precipitation, becoming wetter with elevation. However, rain shadows can occur on sheltered sides of mountains and high plateaus. Highland areas usually derive their annual temperature cycle and the times of their wet and dry seasons from the climate of the surrounding lowland.

7.3 Low-Latitude Climates (Group I): The low-latitude climates lie, for the most part, between the Tropics of Cancer and Capricorn, occupying all of the equatorial zone (10° N to 10° S), most of the tropical zone (10° to 15° N and S), and part of the subtropical zone. The low-latitude climate regions include the equatorial trough of the intertropical convergence zone (ITCZ), the belt of tropical easterlies (northeast and southeast trades), and large portions of the oceanic subtropical high pressure belt. There are four low-latitude climates: 1) wet equatorial, 2) monsoon and trade-wind coastal, 3) wet-dry tropical, and 4) dry tropical.

7.3.1 Wet Equatorial Climate (Köppen: Af). The wet equatorial climate is dominated by warm, moist tropical and equatorial maritime air masses, yielding abundant rainfall year-round. The monsoon and trade-wind coastal climate has a strong wet season that occurs when the ITCZ is nearby.

Suggestion: For fun, mention to the class that the seasonal difference in temperature is so slight that someone was purported to comment that “nighttime is the winter season” in this climate type, or so the legend goes. This “legend” can also be translated to indicate that the daily temperature range is much greater than the annual range, which is in fact correct.

7.3.2 Monsoon and Trade-Wind Coastal Climate (Köppen: Af, Am). In the wet periods of the monsoon and trade-wind coastal climate, equatorial east coasts receive warm, moist air masses from easterly trades, while tropical south and west coasts receive moist air from southwesterly monsoon winds.

Suggestion: Advise students who are fervent supporters of the winter break that, if they can get to the very tip of southern Florida (Miami area) and the Keys, they will actually be in the trade‐wind coastal zone. Those who can only manage Disneyland near Orlando, sorry, you are just too far north in a different climate type. However, since Miami is close to 26º N, there is no guarantee that thermal conditions will be just right during the break. If you want to worry some more, think about how many mountain chains exist in northern Florida that can stop that Canadian air from sweeping all the way down to the southern tip. If you don’t know, fine, we take up that aspect in later chapters.

7.3.3 Wet-Dry Tropical Climate (Köppen: Aw, Cwa). The wet-dry tropical climate has a very dry season alternating with a very wet season. A typical vegetation cover in this climate is savanna woodland, a sparse cover of trees over grassland.

7.3.4 Dry Tropical Climate (Köppen: BWh, Bsh). The dry tropical climate lies in the belt of persistent subtropical high pressure, so rainfall is rare. Temperatures are very high during the high-Sun season but are significantly lower during the low-Sun season.

7.4 Midlatitude Climates (Group II): In midlatitude climate regions, tropical and polar air masses interact, producing traveling cyclones and anticyclones and frontal boundaries. Midlatitude climates range from very wet to very dry and usually show a strong variation in temperature and/or precipitation throughout the year.

7.4.1 Dry Subtropical Climate (Köppen: BWh, BWk, BSh, BSk). The dry subtropical climate resembles the dry tropical climate, but has lower temperatures during the lowSun season, when continental polar air masses can invade the region.

7.4.2 Moist Subtropical Climate (Köppen: Cfa). The moist subtropical climate, found on the eastern sides of continents in the midlatitudes, has abundant precipitation. In summer, maritime tropical air masses provide convective showers and tropical cyclones, while in winter, midlatitude cyclones yield rain and occasional snow.

7.4.3 The Mediterranean Climate (Köppen: Csa, Csb). The Mediterranean climate, found along midlatitude west coasts, is distinguished by its dry summer and wet winter. In summer, dry subtropical high pressure blocks rainfall, while in winter, wave cyclones produce ample precipitation.

Suggestion: Although grapes can be grown in many different locations, many wine “experts” contend that the better wines come from the Mediterranean regions of the world. Taste is in the palate of the specialist and numerous “fun” arguments can easily develop even among good friends.

7.4.4 The Marine West-Coast Climate (Köppen: Cfb, Cfc). The marine west-coast climate features frequent cyclonic storms that provide abundant precipitation, especially when enhanced by an orographic effect. Summers are drier, due to subtropical high pressure that moves poleward, blocking storm tracks.

7.4.5 The Dry Midlatitude Climate (Köppen: BWk, BSk). The dry midlatitude climate occupies continental interiors in rain shadows or far from oceanic moisture sources.

Precipitation is low, and the annual temperature variation is great.

7.4.6 The Moist Continental Climate (Köppen: Dfa, Dfb, Dwa, Dwb). The moist continental climate lies in the polar-front zone, where day-to-day weather is highly variable. Ample frontal precipitation is enhanced in summer by maritime tropical air masses. Winter temperatures fall well below freezing.

7.5 High-Latitude Climates (Group III): By and large, the high-latitude climates occupy the northern subarctic and arctic latitude zones of the northern hemisphere. They also extend southward into the midlatitude zone as far south as about latitude 47° in eastern North America and eastern Asia. The boreal forest climate and tundra climate are absent in the southern

hemisphere, which lacks a high-latitude land mass between 50° and 70° S. However, the ice sheet climate is present in both hemispheres, in Greenland and Antarctica.

7.5.1 Boreal Forest Climate (Köppen: Dfc, Dfd, Dwc, Dwd). The boreal forest climate has long, bitterly cold winters and short, cool summers. For most of the year, cold, dry continental arctic and polar air masses dominate. In summer, occasional maritime air masses provide moisture for precipitation. This climate type has recorded the lowest temperatures in the northern hemisphere in Siberia, with −90º F (−67.8º C) for Verkhoyansk on May 2, 1892 (67.8º N) and Oimekon (63.14º N) on June 2, 1933.

7.5.2 The Tundra Climate (Köppen: ET). The tundra climate occupies arctic coastal fringes. Although the climate is very cold, the maritime influence keeps winter temperatures from falling to the levels of the boreal forest climate. A mild season provides a few months of thaw.

7.5.3 The Ice Sheet Climate (Köppen: EF). The ice sheet climate has the lowest temperatures found on Earth. No month shows mean temperatures above freezing, and winter mean monthly temperatures can fall to –40°C (–40°F) and below.

7.6 Climate Change: Based on a variety of field data, scientists now agree that our planet has experienced a series of climate shifts over the course of its recent geologic history, alternating between colder periods of glaciation and milder interglacial periods such as the present. Over time, plant and animal species adjust to their individual environments, and when those environments change, they must either adapt to, or move to keep pace with, the evolving areas of their older environments in order to survive. During the last glaciation, many common plant species moved toward the Equator and then migrated back during the first few millennia of the interglacial climate. Since the last glaciation ended, approximately 10,000 years ago, the human species has thrived in the current distribution of relatively mild climates. However, Earth is entering a new era of climate change, triggered to a significant degree by increased levels of carbon dioxide (CO2) and other greenhouse gases in our atmosphere, as discussed in detail in prior chapters. Like other species, we, too, will have to adapt to our altered environments.

7.6.1 Shifting Climate Characteristics. There will be great variability in the effects experienced in different regions. Nearly all land regions will get warmer, but with greater variability in temperature and precipitation. Temperature increases are expected to be highest at the poles, due to the circulation patterns that move energy away from the Equator. Based on current trends, scientists expect late-summer arctic sea ice nearly to disappear by the latter part of the century. Global warming will trigger changes in average annual precipitation around the world. Scientists generally expect increased precipitation and evaporation in some areas and drier conditions in others. The prevailing factors that influence global precipitation, such as ocean currents and pressure cells, are shifting, and will shift further.

Suggestion: Discuss recent devastating tornadoes and hurricanes with students, particularly two hurricanes in two years, Irene and Sandy, that have affected the northeastern United States.

7.6.2 Weather Variability. Climate change will affect more than the temperature and precipitation patterns of different climates around the world. The frequency of extreme weather events is also predicted to change. For example, the number of extremetemperature events is forecast to rise in California. In the western mountain ranges, it is expected there will be less precipitation overall, but that winter snowfall extremes will be more likely. Sound evidence also points to an increase in extreme weather events.

Variations in storm paths and intensities reflect variations in major features of the atmospheric circulation patterns.The number of tropical storms and hurricanes varies from year to year, but data show substantial increases in the intensity and duration of these severe storms since the 1970s.

7.6.3 Future Challenges and Adaptations. Over the past few years, climate research scientists have used global climate models to investigate Earth’s past and future climates.

These are complex computer programs that re-create and model the physical, chemical, and biological conditions in Earth’s atmosphere, oceans, land, and ice. Global climate models are calibrated to match Earth’s existing climate by using records from past decades. The capability of these models to re-create our known weather patterns establishes their reliability for future projections. Climate scientists are careful to point out that Earth is much more complicated than the models and mathematical equations they are using to simulate the Earth–ocean–atmosphere system. The big questions facing communities and governments around the world now is how human societies will be affected by climate change, and how they will adapt to that change. For physical geographers and other scientists, the coming decades will prove both professionally challenging and rewarding, as they work to understand and help us all to mitigate the negative effects of environmental change.

Select Audiovisual Aids: Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126). Too Hot Not to Handle: Winning the Battle Against Global Warming. DVD OVW‐362‐44‐KS, 55 minutes, 2006.

Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126). The Science of Climatology. DVD OVW‐300‐70‐KS; VHS OVW‐300‐70‐A, 28 minutes, 2000.

Insight Media, 2162 Broadway, New York, NY 10024‐0621 (800‐233‐9910). Global Climate Regions. DVD, # 79‐AS‐1105, 24 minutes, 2003.

Insight Media, 2162 Broadway, New York, NY 10024‐0621 (800‐233‐9910). Global Warming.

DVD, # 79‐AS‐1131, 46 minutes, 2005.

Select Reference Books: Archer, David. Global Warming: Understanding the Forecast. Wiley, 2006.

Geiger, Rudolph, Robert H. Aron, and Paul Todhunter. The Climate Near the Ground. Rowman and Littlefield, Lanham, MD, 7th ed., 2009.

Guesnerie, Roger and Henry Tulkens (eds.). The Climate of Climate Policy. MIT Press Journals, 238 Main St., Cambridge, MA 02142, 2009.

Pittock, A. Barrie. Climate Change: The Science, Impacts and Solutions. Stylus Publishing, 22883 Quicksilver Drive, Sterling, VA 20166‐2102, 2nd ed., 2009.

United Nations Environment Program. Climate Change Science Compendium, 2009. Bernan Press, Summit, PA (1‐800‐865‐3457).

Wayne, Joseph, David Shearman, and Sandro Positano. Climate Change as a Crisis in World Civilization: Why We Must Totally Transform the Way We Live. Edwin Mellen Press, 415 Ridge St., Lewistown, NY 14092‐0450, 2007.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 8: Biogeographic Processes Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe how energy and matter flow in Earth’s major ecosystems.

2) Explain how organisms interact with their environment and other species.

3) Describe how biotic communities succeed one another.

4) Explain how biogeographic patterns develop over time.

5) Discuss the future of Earth’s biodiversity.

This chapter discusses the processes that control where and when organisms are found at specific locations on planet Earth. It examines the impact of the environment on the spatial distribution of life on Earth, as well as why these spatial distributions change over time. Key topics include: •

Carbon Dioxide —Since the issue of rising atmospheric carbon dioxide has been mentioned with reference to global warming, challenge students to find out more about the carbon cycle. Point students to the Carbon Dioxide Information Analysis Center site and assign them to evaluate changes in atmospheric carbon dioxide at a site of their choice. Consider a discussion to examine sinks and well as sources in the carbon cycle.

Encourage your students to discuss the role of these in managing the carbon cycle.

The nature of biogeographic processes includes many features that students find compelling, especially as many are related to the problems of endangered species and extinction. Many are related to TV series that students may have watched, or you may be able to obtain them on video to show in class.

•

Endangered Species: The importance of the future of biodiversity can be further discussed by examining a particular endangered species. Assign students to choose a plant or animal and investigate its status. Challenge students to identify its position in a food web, and identify the reason for its endangered status.

•

Evolution: A recent PBS TV series focused extensively on the topic of evolution. Assign students to choose a process within evolution and research it using the PBS Evolution site.

•

Virtual Field Trips: There are also a number of virtual field trips such as the Virtual Galapagos site and the Penn State succession site. Assign your students to write an account of a virtual field trip of their choice, while noting the features that they have learned about from lectures and textbook readings.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth's surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are "cached" for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Hubbard Brook Ecosystem Study. A long‐term ecological research project. http://www.hubbardbrook.org/ The Carbon Dioxide Information Analysis Center (CDIAC). An extensive site including current carbon dioxide data and long‐term records. http://cdiac.esd.ornl.gov/ An Introduction to the Nitrogen Cycle. A general site describing the factors associated with the nitrogen cycle. http://muextension.missouri.edu/xplor/envqual/wq0252.htm USDA/NRCS Web Soil Survey provides national coverage of soil maps and related information. http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm Chapter Lecture: 8.1 Energy and Matter Flow in Ecosystems: The distribution of plants and animals on the earth is called biogeography. Ecological biogeography looks at the relationships between organisms and the environment in order to find out where and when these organisms are found. The spatial distribution of the various species and how they have evolved falls under the heading of historical biogeography. The science of ecology examines the interactions between organisms and the environment. The environment in which a group of organisms lives is called an ecosystem.

8.1.1 The Food Web. All ecosystems have food webs that govern the energy flow from primary producers to secondary producers and higher‐level consumers. Decomposers feed on dead plant and animal matter from all levels. It is estimated that only about 10% of the energy is transferred up the food chain from one level to another.

8.1.2 Photosynthesis and Respiration. The production of carbohydrates from water, CO2, and light energy by primary producers is referred to as photosynthesis. The reverse process, whereby carbohydrates are broken down into water and CO2 in order to produce chemical energy that will sustain the life of organisms, is called respiration. Gross photosynthesis is the total amount of carbohydrates that are produced by photosynthesis.

The amount of carbohydrates that remain after enough carbohydrates have been used for power is called net photosynthesis. A simple equation for this process is: Net photosynthesis = Gross photosynthesis – Respiration.

8.1.3 Net Primary Production (NPP). Net Primary Production provides a measure of the rate of carbohydrate accumulation by primary producers. In terms of productivity, the highest levels are equatorial rainforests and wetlands and, as expected, the lowest levels are deserts. The major climatic factors that control NPP are the length of the day, air and soil temperature, and the availability of water.

Suggestion: This is a good place to introduce the concept of orders of magnitude to the class.

Basically, going from 1 to 10 is one order of magnitude; going from 1 to 100 is 2 orders, and so on. The significance of this progression is that the increase (or decrease) is exponential. Just look at the average NPP of selected ecosystems in g/m2/yr. It ranges from 3 in very dry deserts to 360 in continental shelf locations (over 2 orders of magnitude: 3 to 30 to 300) to 2,500 in fresh water swamps and marshes (way over 3 orders of magnitude: 3 to 30 to 300 to 2,500).

8.1.4 Biogeochemical Cycles. Atoms and molecules move through ecosystems under the influence of both physical and biological processes. We call the pathways that a particular type of matter takes through Earth’s ecosystem a biogeochemical cycle (sometimes referred to as a material cycle or nutrient cycle ). Two of the most important biogeochemical cycles are the carbon cycle and the nitrogen cycle. I. The Carbon Cycle: The Carbon Cycle is one type of biogeochemical cycle through which carbon can flow among storage pools that exist in the atmosphere, ocean, and on the land. The substantial increase in fossil fuel combustion over the past several centuries has affected the carbon cycle, resulting in an increase in CO2 concentrations in the atmospheric storage pool.

II. The Nitrogen Cycle: The Nitrogen Cycle is another biogeochemical cycle that has a huge storage pool in the atmosphere. Note that 78% by volume of the atmosphere consists of N2, which cannot be used directly by most organisms. Nitrogen fixation can occur when N2 is converted into more useful forms by bacteria or blue‐green algae. The rate of nitrogen fixation has doubled as the result of anthropogenic activities, such as the manufacture of fertilizer.

8.2 Ecological Biogeography: This section looks at how plants and animals are distributed, based on various ecological factors, and the interrelationships among their species. The habitat of an organism, or a community of organisms, occurs within a particular environment. Each species has an ecological niche that describes its energy source, the nature of its energy sources, and its influence on other species. A group of interacting organisms that occupy a particular habitat is called a community.

8.2.1 Water Need. Water need affects both plants and animals because it is so critical for life. Xerophytes are plants that can survive in dry areas. Certain plant types (phreatophytes) have deep roots that can extract water from groundwater sources. Other plants (succulents) are able to store water in spongy tissues within the plant. Certain climate types, such as the Mediterranean, are dry during the high‐Sun period and wet during the low‐Sun period. Plants (sclerophylls) that can survive this strong seasonal wet‐dry alternation develop hard, thick, leathery leaves.

In order to survive, some species of cactus are surrounded by bare ground so as to maximize their chances of getting some water when there is a period of rain, however brief. In other words, they all can’t survive if they are too close. Mammals are not well

adapted to dry conditions. In order to cope with limited water in their environment, xeric animals adopt certain techniques such as becoming dormant during dry spells and nesting during rainy periods.

Suggestion: Ask the class why a certain corporation in Rochester, NY decided to call its new product “Xerox.” It is presumed that those who have made photocopies will note that the process is dry.

8.2.2 Temperature. Temperature has a direct influence on the physiological processes that occur in plant and animal tissues. Each plant species has an ideal temperature that goes best with its various functions. These include photosynthesis, flowering, fruiting, or seed germination. colder climates have fewer species of plants and animals. Cold-blooded animals tend to be active only during the warmer parts of the year because they lack a mechanism for regulating temperature. In contrast, warm-blooded animals have developed a variety of methods in order to maintain a relatively constant body temperature.

8.2.3 Other Climatic Factors. Additional climate factors include light intensity, the length of the daylight period and growing season, and the duration and intensity of the wind, all acting together to determine the distribution patterns of plants and animals.

Consequently, biogeographers can delineate a bioclimate frontier—a geographic boundary that can mark the limits on a map of the potential distribution of a species.

8.2.4 Geomorphic Factors. Geomorphic, or landform, factors, such as slope steepness, slope angle and aspect (northern or southern exposure), and relief (difference between higher and lower elevations), all act to influence the distributions of plants and animals.

8.2.5 Edaphic Factors. Edaphic factors are soil‐related, such as the size of the soil particles and the amount of organic matter in the soil. These factors affect the distribution patterns of organisms due to the fact that there is a tremendous heterogeneity in soil types, even within a small area.

Suggestion: Depending upon the size of the class, it would be helpful to show a soils map for a county in your area, showing the enormous variation in soil types, to the students. You might also have the students access the USDA/NRCS Web Soil Survey site and locate a particular area of interest (see “Relevant Internet Resources” section above).

8.2.6 Disturbance. Disturbance factors include fires, floods, storm waves, and damaging winds. Specialized species in many ecosystems can become adept at adapting to such disturbances.

8.2.7 Interactions Among Species. Species’ interaction can be positive or negative.

Positive interaction among species is called symbiosis and is exemplified by commensalism, protocooperation, and mutualism. Examples of negative interaction include competition for resources, predation, parasitism (when a parasite enters the body

of a host), herbivory (overgrazing of a food source), and allelopathy (when a plant produces a poison a chemical toxin that poisons the soil against competing species).

8.3 Ecological Succession: The process whereby plant communities succeed one another as they move to some form of stability is referred to as ecological succession. There can be several stages prior to a climax, or final stage, of ecological succession.

8.3.1 Succession, Change, and Equilibrium. These processes describe the inherent tendency over time for ecosystems to change. These changes are subject to both natural and anthropogenic environmental disturbances such as high winds, flood, fire, and logging.

8.4 Historical Biogeography: This section looks at spatial distribution patterns of organisms over long time periods at continental and global scales.

8.4.1 Evolution. Evolution as a process has allowed life on Earth to reach unbelievable levels of diversity and complexity. In addition, the number of species of plants and animals that have been identified and described probably represents only a very small fraction of all of the species that actually exist in the world. The evolutionary process is based on natural selection, which involves the survival and reproduction of the fittest of the species. Thus, natural selection utilizes variation to result in populations that are better adjusted to their environment over time.

Suggestion: Mention to the class that both Charles Darwin and Alfred Wallace independently summarized their ideas about evolution in 1858, but Darwin published his detailed Origin of Species in 1859 and was duly recognized for this outstanding contribution.

8.4.2 Speciation. Speciation is the process by which species are differentiated and maintained. Mutation, natural selection, genetic drift, and gene flow are its component processes.

8.4.3 Extinction. Extinction occurs when conditions in the environment change at a greater rate than a species can evolve new adaptations to survive. Rare but extreme events, such as the meteorite that struck Earth about 65 million years ago, eliminated the largest land animals that ever existed on this planet (dinosaurs), in addition to many other land and sea species. The size of the Chicxulub crater in the Yucatan Peninsula of Mexico is estimated to be more than 110 miles (177 km) in diameter with the impacting meteorite having an estimated diameter of 6 mi (9.7 km).

Suggestion: Mention to the class that if they want to get a good idea of what a crater looks like, albeit a much smaller one than Chicxulub, a good example is Meteor Crater (a.k.a. Barringer Crater) in Arizona. For convenience, it is just off I‐40 between Flagstaff and Winslow in the northern part of the state. No, you can’t go wandering around at the base of the crater, but there is an excellent observation deck at the rim where you can observe the depth and width of this much more recent, but much smaller, visitor from outer space. The crater is about 4,000 ft (1,219

m) in diameter with a depth of about 600 ft (183 m). Its age is estimated to be somewhere between 5,000 and 50,000 years ago (the age most commonly given is ca. 50,000 years ago).

8.4.4 Dispersal. Dispersal is the capacity to move from a location of origin to new sites. In diffusion, species extend their range slowly from year to year. In long-distance dispersal, unlikely events establish breeding populations at remote locations.

8.4.5 Distribution Patterns. Distribution patterns of species include the following types: 1) Endemic species that are found in only one location, 2) Cosmopolitan species that are widely distributed across Earth (such as humans), and 3) Disjunction, which occurs when one or more closely related species turn up in distant regions.

8.4.6 Biogeographic Regions. Biogeographic regions occur when closely related species are found in similar areas, where they have common evolutionary histories and environmental preferences.

8.5 Biodiversity: Biodiversity refers to the variety of biological life on Earth. Numerous plant and animal extinctions are related to human activity, and this trend seems to be continuing at various locations across the planet. Additional contributing factors include unbridled population growth and poor management practices of land and water resources in many parts of the world.

Maintaining biodiversity is extremely important because nature has provided an incredibly rich array of organisms that interact with one another in a seamless web of organic life. When we cause the extinction of a species, we break a link in that web. Ultimately, the web begins to fray, with unknown consequences for both the human species and all other forms of life on Earth.

Suggestion: Discuss some fairly recent extinctions. For example, the moa was a large (some reached 10 ft [3 m]), flightless, slow‐growing bird in New Zealand that was presumably hunted to extinction by the Maoris, who arrived at the islands about 700 years ago. A more recent example is the American passenger pigeon, which lived only in eastern North America. It was considered the most abundant bird in the world, with an estimated population of 3 to 5 billion.

Indeed, John Audubon stated in 1830 that he saw one enormous flock of these birds that was 10 miles (16.1 km) wide and hundreds of miles long, estimated to contain nearly a billion birds.

Habitat destruction and excessive hunting severely reduced the population between 1870 and 1890, and by 1900, the last known passenger pigeon had been killed.

Select Audiovisual Aids: I found no videos specifically related to biogeographic processes.

Select Reference Books: Cunningham, William P. and Mary Ann Cunningham. Principles of Environmental Science, 4th ed., New York: McGraw‐Hill, 2008.

Kaufmann, Robert K. and Cutler C. Cleveland. Environmental Science, New York: McGraw‐Hill, 2008.

Merchant, Carolyn. American Environmental History: An Introduction. Irvington, NY: Columbia University Press, 2007.

Schmitz, Oswald J. Ecology and Ecosystem Conservation. Washington, DC: Island Press, 2007.

Trudgill, Stephen. The Terrestrial Biosphere: Environmental Change, Ecosystem Science, Attitudes and Values. New York, Prentice Hall, 2001.

Waltner‐Towes, David, James J. Kay, and Nina‐Marie E. Lister. The Ecosystem Approach: Complexity, Uncertainty, and Managing for Sustainability. Irvington, NY: Columbia University Press, 2008.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 9: Global Biogeography Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe the types of plant cover.

2) Identify the principal biomes.

3) Identify the major formations of the forest biome.

4) Compare savanna and grassland biomes.

5) Compare desert and tundra biomes.

6) Describe how biomes change with climate and altitude.

Global ecosystems include many regions that offer interesting and compelling examples. The systematic and sequential nature of vegetation types can make it difficult to avoid teaching and learning by rote. Here are a few suggestions on how to get this information across.

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Spread it out—Rather than teaching all the biomes as one block, introduce one or two of the biome types throughout the course, perhaps with relevant soils and climate.

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Compare and contrast—Teach two or three biome types together to compare them to one another, for example savanna and grassland.

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Explore biomes using the web sites—From a list of locations provided by the instructor, have students evaluate the biome of a place, using the World Biome.com site. Challenge students to identify which features of the biome type are evident in the biome they select.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Mission Biomes (NASA). This site explains the use of satellite imagery to monitor the Earth’s biomes. http://earthobservatory.nasa.gov/Laboratory/Biome/ The Wilds of Madagascar. A Nova site looking at the many aspects of the environment of Madagascar. http://www.pbs.org/wgbh/nova/madagascar/explore/

Animal Diversity Web from the University of Michigan Zoology Museum. http://animaldiversity.ummz.umich.edu/ Amazon Interactive. A multimedia tour of the Amazon basin, with particular reference to biodiversity. http://www.eduweb.com/amazon.html Virtual Fieldtrip to Indian Peaks, Colorado. An introduction to altitudinal zonation of vegetation and the effects of glaciation. The fieldtrip travels from mixed aspen, to tundra transition landscapes to active glaciers. http://www.uwsp.edu/geo/projects/virtdept/ipvft/ipvftmod.html Major Biomes of the World. A general introduction to the world’s biomes with numerous pictures. https://php.radford.edu/~swoodwar/biomes/ World Biomes.com. Another general introduction to the world’s biomes with many pictures. http://www.worldbiomes.com/ Chapter Lecture: 9.1 Natural Vegetation: Plant cover that develops with minimal human intervention is referred to as natural vegetation. Human activities, particularly in the midlatitudes, extensively modify the landscape by suppressing fire, introducing new species, using intensive agriculture, grazing, and facilitating urban sprawl.

9.1.1 Structure and Life‐Form of Plants. Structure and life‐form of plants refer to a plant’s physical structure, size, and shape. Lichens, herbs, lianas (such as poison ivy), shrubs, and trees are all different types of life‐forms.

9.2 Terrestrial Ecosystems —The Biomes: The largest unit of terrestrial ecosystems is the biome, which includes forest, grassland, savanna, desert, and tundra. Vegetation structure and life‐form facilitate the placement of formation classes as subdivisions of biomes.

9.2.1 Biomes, Formation Classes, and Climate. The pattern of formation classes depends heavily on climate. As climate changes with latitude or longitude, vegetation will also change. In both low- and midlatitude environments, strong precipitation gradients produce vegetation types grading from forest to desert. At high latitudes, decreasing temperatures control the transition from forest to tundra. In low latitudes, savanna and grassland formation classes are found in regions with a distinct dry season. In the midlatitudes, west coasts are marked with a strong summer dry period, providing sclerophyll vegetation in coastal regions and, farther poleward, encouraging the growth of lush coastal forests of conifers.

9.3 Forest Biome: Within the forest biome, we can recognize six major formations: low-latitude rainforest, monsoon forest, subtropical evergreen forest, midlatitude deciduous forest, needleleaf forest, and sclerophyll forest. Ecologists also sometimes recognize three principal types of forest as separate biomes, based on their widespread nature and occurrence in different

latitude belts: low-latitude rainforest, midlatitude deciduous and evergreen forest, and boreal forest.

9.3.1 Low‐Latitude Rainforest. Low‐latitude rainforests are characterized by warm temperatures and a generally wet climate, although there may be a short dry season.

There is a substantial diversity with very large numbers of plant and animal species. The vegetation cover is dominated by broadleaf evergreens.

9.3.2 Monsoon Forest. Monsoon forests are open covers of deciduous trees that experience leaf‐shedding during dry seasons that can be stressful. This formation class is found in wet‐dry tropical climates.

9.3.3 Subtropical Evergreen Forest. Subtropical evergreen forests are associated with the moist subtropical climates of the southeastern parts of North America and Asia. A large portion of this formation class is no longer cultivated.

9.3.4 Midlatitude Deciduous Forest. Midlatitude deciduous forests are located in eastern North America (humid continental climate) and western Europe (marine west‐coast climate).They are mostly comprised of deciduous trees that lose their leaves in the winter.

9.3.5 Needleleaf Forest. Needleleaf forests are composed primarily of evergreen conifers.

This category includes the coastal forest of the Pacific Northwest and the boreal forest in North America and Eurasia between 45º and 75º N latitude.

9.3.6 Sclerophyll Forest. Sclerophyll forests are associated with the Mediterranean climate, which has a long dry‐season drought period. Accordingly, the dominant trees are low with leathery, thick leaves that are well adapted to this unusual type of seasonal precipitation.

9.3.7 Deforestation. Since humans took up agriculture, Earth’s forests have been diminishing through the process of deforestation. At present, about 13 million hectares (32 million acres) of forest are lost each year, largely to agriculture. More than half of this area is in South America, Africa, and equatorial Asia. Much of the deforestation can be linked to economic development policies that promote cultivation of cash crops such as soybeans and palm oil. Loss of habitat in these species-rich areas takes a toll on the planet’s biodiversity. Slashing and burning of forest also releases carbon dioxide, and loss of evapotranspiration from trees leads to less rainfall and higher temperatures.

9.4 Savanna and Grassland Biomes: The savanna and grassland biomes of African safaris and Argentine gauchos support an important variety of grazing mammals and their predators, as well as the world’s great migrating herds. Unfortunately, these lands are easily converted to agriculture when irrigated by water drawn from deep wells, and this practice hastens the loss of these unique biomes.

9.4.1 Savanna Biome. Savanna biomes are composed of trees that are widely spaced with a grassy understory. This biome is associated with the tropical wet‐dry climate of

Africa and South America. Frequent fires during the dry season reduce the amount of trees and encourage more grasses. Large numbers of grazing animals and their predators (lions, leopards, cheetahs, hyenas, and jackals) are characteristic of the African savanna.

4.2. Grassland Biome. Grassland biomes in the midlatitude regions include the formation classes of tall‐grass prairie in wetter environments and short‐grass prairie (or steppe) in drier areas. In a similar manner to the savanna biome, fire is an important part of the grassland biome. The tall‐grass prairies are suitable for crop cultivation, whereas short‐grass prairies are better suited for grazing.

9.5 Desert and Tundra Biomes: The desert is a highly evolved ecosystem that supports a multitude of plants and animals. Insects, reptiles, mammals, and birds can occasionally be spotted at night when the Sun ceases its unfiltered radiation of the sparse vegetation. Rare and fantastic plants may flower after many decades, when sufficient rain finally falls, triggering the germination of long-dormant seeds. And lucky are those who see them, for these desert blooms may last only a few days or weeks.

9.5.1 Desert Biome. Desert biomes are found in the dry tropical, dry subtropical, and dry midlatitude climate zones. The basic formation classes are semi‐desert and dry desert.

Plant types include xerophytes, thorny shrubs, and small trees.

9.5.2 Tundra biome: The tundra biome is associated with the cold, dry tundra climate on the fringes of the Arctic Ocean. Plants (mostly low herbs and shrubs) are low to the ground and have adapted to the severe drying cold and frost.

9.6 Climate and Altitude Gradients: The climatic factors of temperature and precipitation have obvious variation as elevation and space change. As a result, altitude and systematic variation on transects of great length usually show patterns of vegetation that vary latitudinally and longitudinally.

9.6.1 Climate Gradients and Biome Types. Because climate factors of temperature and precipitation vary with elevation and over space, vegetation patterns often show zonation with altitude and systematic variation on long transects.

9.6.2 Altitude Gradients. Climate varies not only with latitude and longitude, but also with elevation. At higher elevations, temperatures are cooler and precipitation is usually greater. This can produce a zonation of ecosystems with elevation that resembles a poleward transect.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (800‐255‐8629). Our Green Planet, DVD # DV‐3152, 17 minutes.

Crystal Productions, Glenview, IL (800‐255‐8629). Nature’s Delicate Balance. DVD # DV‐3147, 15 minutes.

Films for the Humanities and Sciences. Hamilton, NJ (800‐257‐5126). Grassland Biomes. DVD OUR 297‐72‐K; VHS OUR 297‐72‐A, approx. 20 minutes, 2002.

Films for the Humanities and Sciences. Hamilton, NJ (800‐257‐5126). Rainforest Biomes. DVD OUR 297‐73‐K; VHS OUR 297‐73‐A, approx. 20 minutes, 2002.

Insight Media, New York, NY (800‐233‐9910). Biomes. DVD # 79‐AR‐1254, 89 minutes, 2003.

Insight Media, New York, NY (800‐233‐9910). Terrestrial Biomes: Deserts, Grasslands, and Forests. DVD #79‐AR‐1288, 37 minutes, 2005.

Select Reference Books: Ahern, Jack, Elizabeth Leduc, and Mary L. York. Biodiversity Planning and Design: Sustainable Practices. Washington, DC: Island Press, 2006.

Allen, Craig R. and C.S. Holling (eds.). Discontinuities in Ecosystems and Other Complex Systems. Irvington, NY: Columbia University Press, 2008.

Cunningham, William P. and Mary Ann Cunningham. Principles of Environmental Science.

New York: McGraw‐Hill, 4th ed., 2008.

Hobbs, Richard J. and Katherine N. Suding (eds.). New Models for Ecosystem Dynamics and Restoration. Washington, DC: Island Press, 2008.

Jarvis, D.I., C. Padoch, and D. Cooper (eds.). Managing Biodiversity in Agricultural Ecosystems.

Irvington, NY: Columbia University Press, 2007.

Kaufmann, Robert K. and Cutler C. Cleveland. Environmental Science, New York: McGraw‐Hill, 2008.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 10: Global Soils Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe the properties used to describe soils.

2) Identify chemical properties of the soil and the chemical processes that change soils.

3) Describe the variation of moisture in the soil.

4) Explain how soil layers develop distinctive characteristics.

5) Describe how soil types are distributed around the world.

It can be very difficult to make soils interesting to students, who often see the subject as just dirt.

Here are a few suggestions to improve the class’s learning experience of this significantly important topic.

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Soil classification—As the soil classification scheme is systematic and organized, students may attempt to study soil types by rote memorization. This can make the topic seemingly dull and also difficult. Images, such as those found on the NRCS Soils site, help bring the subject matter to life.

•

Spread it out—Rather than teaching all the soils as one block, introduce one or two of the soil types throughout the course, perhaps with relevant climates and vegetation.

•

Compare and contrast—Teach two or three soil types together and compare them to one another, for example, inceptisols and entisols.

•

Explore soils using web sites —Have students identify and describe the soil of a particular location, such as that found at their home, using the Web Soil Survey.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Soil Taxonom —the Twelve Soil Orders. A brief review of the features of the twelve soil orders. http://soils.ag.uidaho.edu/soilorders/

Canadian Soil Information System. An explanation of the system used to classify Canadian soils. http://sis.agr.gc.ca/cansis/intro.html Soil Science Society of American online soil glossary. A detailed reference explaining the many terms associated with soil science. https://www.soils.org/publications/soils-glossary/ USDA/NRCS Soils Website. This site contains a wealth of information about soils, with a particular emphasis on soils of the United States. http://soils.usda.gov/ USDA Web Soil Survey. This site lets the user access detailed soil’s information from all areas of the United States. http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm National Soil Database of Canada. A detailed reference that records and examines the extent of different soil types in Canada. http://sis.agr.gc.ca/cansis/nsdb/intro.html The Pedosphere and Its Dynamics. A general explanation of the soil and its systems. http://www.pedosphere.com/ The World Wide Soil Jump Station. An extensive series of links to a wide variety of web pages about soils. http://soil.hostweb.org.uk/ Best Management Practices for Soil Erosion (EPA). Download this self-extracting file to see a general introduction on soil erosion. http://www.epa.gov/oecaagct/ag101/cropsoil.html Chapter Lecture: 10.1 The Nature of the Soil: The uppermost layer of the land surface is soil. All plants need soil for nutrients, water, and physical support. Parent material, climate, vegetation, relief (topography), and time are the major factors that affect soil and its development.

10.1.1 Introducing the Soil. Soil is a complex mixture of solids, liquids, and gases. The solid matter includes both mineral and organic matter. Soils also contain atmospheric gases and watery solutions. A vertical cross section of the soil shows an uppermost layer of vegetation and forest litter (when available at the sample site), then a layer of soil at various depths, followed by regolith, consisting of weathered bedrock, and finally the bedrock at depth.

10.1.2 Soil Color and Texture. Soil color and texture include a range of colors from white (mineral salts in dry climates), red and yellow (iron‐containing oxides), to dark brown and black (an abundance of organic constituents characteristic of prairie soils).

The proportion of sand, silt, and clay in a soil is referred to as soil texture. Soil texture is a very important attribute because it regulates the amount of water the soil can retain.

Suggestion: Collect several samples of local soils, preferably with a variety of textures. Have students go to the USDA/NRCS website. Search for and print a copy of the Guide to Texture by

Feel flowchart. Have the class break into groups and attempt to determine the texture of each soil sample using this flowchart. Bring plenty of paper towels because this is a messy exercise.

10.1.3 Soil Structure. Soil structure pertains to the manner in which soil grains become grouped together into larger masses that are referred to as peds. They vary in size from small grains to larger blocks. Common soil structures include granular, platy, blocky, and columnar. Cultivation is much easier when the soils have granular or blocky structures.

10.2 Soil Chemistry: In addition to physical properties such a soil texture and structure, soil chemistry and soil moisture are important factors that determine the suitability of soil to support vegetation. The chemistry of a soil determines how well it can provide nutrients to plants. The availability of soil moisture determines the capability of the vegetation to grow and thrive.

10.2.1. Acidity and Alkalinity. One important element in soil chemistry is pH, which is a measure of how acidic or alkaline the soil is. Soil pH is an important indicator of soil fertility. It affects how minerals dissolve in water solutions and their availability for uptake by plant roots. Soil acidity varies widely. Soils of cool, moist regions are generally acidic, while soils of arid climates are alkaline. Acidic soils are often low in base ions.

10.2.2 Soil Colloids. oil colloids are particles smaller than one ten-thousandth of a millimeter (0.0001 mm [0.000004 in.]). Like other soil particles, some colloids are mineral, whereas others are organic. Soil colloids are important because their surfaces attract soil nutrients dissolved in soil water as positively charged mineral ions, or cations.

10.2.3 Mineral Alteration. Primary minerals in soils are those that remain from unaltered rock. Secondary minerals are formed by mineral alteration. Clay minerals and sesquioxides are secondary minerals that are very important in soil development and fertility.

10.3 Soil Moisture: The soil layer is also a reservoir of moisture for plants. Soil moisture is a key factor in determining how the soils of a region support vegetation and crops. The interrelationships among soil moisture, climate, and vegetation help us understand how changes in climate may affect agriculture and native ecosystems.

10.3.1 Soil-Water Storage. Soil-water storage starts with precipitation. Some of the water runs off the land surface and winds up in streams that eventually reach the oceans.

Another portion of the water returns to the atmosphere as water vapor from soil water evaporation and plant transpiration—a combined process called evapotranspiration. The water remaining in the soil can cling to the soil particles by capillary action. The ability of soils to hold water after the excess water has drained away depends on soil texture, which in turn defines the storage capacity of the soil. Sandy soils have a lower storage capacity than clayey soils. The wilting point of soils is a function of soil texture as it represents the level of water storage in the ground below which plants will start to wilt.

10.3.2 Soil‐Water Balance. The soil-water balance represents the gain and loss of water in storage in the soil. Precipitation, evapotranspiration, and runoff are the governing agents in the overall balance. Depending on the balance, soil moisture may be lowered or recharged.

10.4 Soil Development: 10.4.1 Soil Horizons. Soil horizons are distinctive horizontal layers at varying depths in the soil that have different physical and chemical characteristics, organic contents, and/or structure. The layered structure of a particular soil can be shown by a vertically arranged soil profile.

10.4.2 Soil‐Forming Processes. Soil-forming processes are divided into four classes: 1) enrichment—occurs when organic (such as humus) or inorganic (such as minerals) material is added to the soil, 2) removal—occurs when erosion physically transports soil particles into streams and by leaching, whereby soil compounds and minerals in solution move to lower levels, 3) translocation—occurs when fine particles are moved downward by eluviation (“washed-out of”) and build up in lower horizons by illuviation (“washed into”), 4) transformation—occurs when primary minerals are converted to secondary minerals. Transformation also includes humification, a process whereby bacterial decay breaks down organic matter into humus.

10.4.3 Factors of Soil Formation. The five principal factors of soil formation include: climate relief, organisms, parent material and time. Human activity, such as farming and construction, can also be considered an important factor influencing soil formation. Soil temperature and other factors assist in determining the formation of soil horizons and the chemical development of soils. A warm climate facilitates the rapid decomposition of organic matter that can result in a scarcity of this valuable material. In contrast, cold temperatures result in slow decomposition and consequent accumulation of organic matter.

10.5 The Global Scope of Soils: The soils of the world have been classified by the U.S. National Resources Conservation Service (formerly the Soil Conservation Service) into four major groups consisting of 12 orders. The divisions are based primarily on the unique physical/ chemical presence of certain diagnostic horizons that display dominant factors of maturity, climate, parent material, and high organic content.

10.5.1 Soil Characterized by Maturity. This group includes the following soil orders: Entisols, Inceptisols, Alfisols, Spodosols, Ultisols, and Oxisols. Entisols lack horizons, often because they are only recently deposited. They may occur in any climate or region.

Inceptisols have only weakly developed horizons. Inceptisols of river floodplains and deltas are often very productive. Alfisols have horizons of eluviation and illuviation of clays. They also have a high base status and can be very productive. They are associated with moist climates that range from equatorial regions to subarctic zones. Spodsols have

a light-colored albic horizon of eluviations and a dense spodic horizon of illuviation.

They develop under cold, needleleaf (boreal, coniferous) forests and are quite acidic.

Oxisols and Ultisols develop over long time periods in warm, moist climates. Oxisols have substantial accumulations of iron and aluminum sesquioxides. Ultisols have a horizon of clay accumulation.

10.5.2 Soils Characterized by Climate. This group includes the following soil orders: Mollisols, Aridisols, and Gelisols. Mollisols are soils of grasslands in subhumid to semiarid climates. They have a thick, dark brown surface layer, termed a mollic epipedon. Because of their loose texture and high base status, they are highly productive.

Aridisols are desert soils with weakly developed horizons. They often exhibit subsurface layers composed of an accumulation of calcium carbonate or soluble salts. With irrigation and proper management, they are quite fertile. Gelisols are soils of permafrost regions that are churned by freeze/thaw ice action.

10.5.3 Soils Characterized by Parent Materials. This group includes the following soil orders: Vertisols and Andisols. Vertisols develop on certain types of volcanic rock in wet-dry climates under grassland and savanna vegetation. They expand and contract with wetting and drying, creating deep cracks in the soil. Andisols are unique soils that form on volcanic ash of relatively recent origin. hey are dark in color and typically fertile.

10.5.4 Soils High in Organic Matter. Histosols are organic soils, often termed peats or mucks. They are typically formed in cool or cold climates in areas of poor drainage.

Suggestion: Mention to the class the value of peat as a source of fuel in areas that are deficient in coal, such as Ireland. Although the calorific value of peat is less than that of coal, it is better than nothing. Of course, the peat will eventually turn into coal, but you will have to wait a good long time for that to occur.

Select Audiovisual Aids: Films for the Humanities and Sciences, Hamilton, NJ (800‐329‐5126). The Once Good Earth: Understanding Soil. DVD OVW 316‐91‐KS; VHS OVW 316‐91‐A, 46 minutes, 2005.

Insight Media, New York, NY (800‐233‐9910). Introduction to Soil Science: Soil Forming Factors. DVD #79‐AS‐765, 25 minutes, 1998.

Insight Media, New York, NY (800‐233‐9910). Properties of Soils. DVD #79‐AR‐746, 24 minutes, 2002.

Select Reference Books: Certini, Giacomo and Richard Scalenghe. Soils: Basic Concepts and Future Challenges.

Cambridge University Press, New York, NY, 2006.

Charman, Peter and Brian Murphy. Soils: Their Properties and Management. Oxford University Press, New York, NY, 3rd ed., 2007.

Gerrard, John. Fundamentals of Soils. Routledge, New York, NY, 2000.

Schaetzl, Randall J. and Sharon Anderson. Soils: Genesis and Geomorphology. Cambridge Univerity Press, New York, NY, 2005.

Singer, Michael J. and Donald N. Munns. Soils: An Introduction. Upper Saddle River, NJ: Prentice Hall, 4th ed., 1999.

Toy, Terrence J., George R. Foster, and Kenneth G. Renard. Soil Erosion: Proceses, Prediction, Measurement, and Control. Wiley, 2002.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 11: Earth Materials and Plate Tectonics Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe key events in Earth history.

2) Describe the Earths’ structure from the surface to the center.

3) Identify types of rocks found on Earth’s surface.

4) Describe the features of Earth’s continents and ocean basins.

5) Describe the types of tectonic processes and their results.

Understanding rocks and minerals is important in understanding geomorphology, which comprises such a large proportion of physical geography. Students may be tempted to learn some rock and mineral characteristics by rote, but this does not aid in their understanding of related topics, such as land forms. The nature of plate tectonics is important for students to understand volcanoes, earthquakes, and the morphology of the planet. Simply memorizing the structure of Earth and the geologic time scale does not promote significant understanding.

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Origins of Rocks: A general explanation of igneous, sedimentary, and metamorphic rocks allows students to focus on the characteristics of rocks, such as resistance to erosion. The Igneous, Metamorphic, and Sedimentary Rocks site from UBC provides a useful introduction. Point students to the Clickable Rock Cycle web site to examine the processes involved.

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Minerals: The large array of minerals is beyond the realm of the text; however, the rocks and minerals interactivity in WileyPLUS shows many of the most common minerals.

Web sites such as the Mineral Gallery provide further information.

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Challenge students to identify rocks and minerals of the local region and access Mineral Gallery to find out more about them.

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Develop a list of famous geologic structures and monuments and challenge students to identify the rocks involved (e.g. Ayers Rock, Australia (Uluru), Mount Rushmore, the Great Pyramids).

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Fossils: There is also a vast array of different fossils, and these too can interest students.

Point students toward sites such as the Fossils, Rocks and Time site. As with local rocks and minerals, where possible, challenge students to identify local fossils and create a chronology of local rock strata.

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Geology in the Landscape: As a means to prepare students for the important role of Earth materials in geomorphology, point them to sites such as A Geologist’s Lifetime Field List or Geology by Light Plane to demonstrate the role of rocks in the landscape.

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Challenge students to create their own “lifetime lists” for physical geography.

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Present pictures and souvenirs from trips you have taken that evidence the variety of Earth’s geologic features.

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The Structure of Earth: As the structure of Earth is described by its different layers, it can be all too tempting for students to simply memorize the individual layers rather than fully understand the topic. Provide students with a selection of questions and point them to the This Dynamic Earth site by the USGS to investigate the answers.

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Plate Tectonics and Continental Drift: The slow and gradual nature of continental drift can make this topic seem obscure and distant to students. A number of web sites provide innovative ways to enliven this subject. Point students to the Plate Motion Calculator site and assign them the task of calculating how fast a specific plate is moving. Compare the rate of motion to other phenomena (see the section on relative scales on the Chapter 2 web site). Although designed for children, the NASA Pangaea Interactive Map Game is a useful guide to labeling the continents of Pangaea. For students who know nothing about continental drift, this may a good preparatory exercise before attempting the interactivity in WileyPLUS.

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The Geologic Time Scale: The immensity of the geologic time scale can prove daunting for beginning students. There are numerous web sites, such as the Comprehending Geologic Time Calculator and Berkeley’s Geological Time Machine, that give students a clear picture of how and when Earth has experienced some of its greatest changes.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

The Mineral Gallery. An extensive list of minerals with photographs and characteristics. http://mineral.galleries.com/ Collectors Corner. A useful site from the Mineralogical Society of America to guide people in collecting their own rocks and minerals. http://www.minsocam.org/msa/collectors_corner/index.htm

Mineral Information Institute. An extensive site with educational programs for schools on the role of mineral industries. http://www.mii.org/ The Burgess Shale: A Hidden Treasure in the Canadian Rockies. An important example of a fossil‐rich strata with ancient animals. http://park.org/Canada/Museum/burgessshale/titlen.html Fossils, Rocks and Time (USGS). A general introduction to fossils and the geological time scale from the US Geological Survey. http://pubs.usgs.gov/gip/fossils/ Metamorphic Rock Identification. A guide to the characteristics and qualities of different types of metamorphic rocks. http://geology.csupomona.edu/drjessey/class/Gsc101/Meta.html Geology by Light Plane. Professor Louis Maher, Jr. provides a selection of the images he has taken from the air to show a variety of geologic features and landforms. http://www.geology.wisc.edu/~maher/air.html This Dynamic Earth ‐ The Story of Plate Tectonics. A thorough introduction to the discovery and theories of plate tectonics. http://pubs.usgs.gov/publications/text/dynamic.html The Interior of the Earth (USGS). A brief introduction to the structure of Earth and related processes. http://pubs.usgs.gov/gip/interior/ PaleoMap Project. An extensive ongoing research project that is accurately reconstructing the shape and positions of the continents through geologic time. http://www.scotese.com/ NASA Pangaea Interactive Map Game. A simple game to label the continents evident in Pangaea. http://kids.earth.nasa.gov/archive/pangaea/Pangaea_game.html Dive and Discover ‐Expeditions to the Sea Floor. A well-illustrated introduction to tectonic landforms and features on the ocean floor. http://www.divediscover.whoi.edu/ Descent to the Mid‐Atlantic Ridge. A detailed description of the Mid‐Atlantic ridge derived from modern submarine technologies. http://earthguide.ucsd.edu/mar/ Land and Sea Topography. Satellite-derived full‐color images of relief on land and in the ocean basins. http://www.ngdc.noaa.gov/mgg/image/2minrelief.html Geological Time Machine. A detailed description of each geologic time period with reference to environmental conditions and the biosphere. http://www.ucmp.berkeley.edu/help/timeform.html Comprehending Geologic Time Calculator. It’s difficult to envision the long periods of time associated with the geologic time scale. This calculator allows you to compress it into something more familiar and manageable, such as a calendar year. http://www.athro.com/geo/timecalc.html

Timelines at the American Museum of Natural History. A useful exhibition of different types of fossils associated with different geologic time periods. http://www.amnh.org/content/search?SearchText=fossil+ahll+timeline Chapter Lecture: 11.1 The Changing Earth: This chapter begins the section of the book dedicated to geomorphology, the study of the shape of Earth’s features and how they change over time.

Geographers build upon the foundations of geomorphology to understand the differences in the distribution of humans, plant and animal species, and natural resources. At broader scales, the shape of Earth’s surface features depends on the underlying rocks, so we begin our study with a close look at Earth’s inner structure, materials, and global topography. To reconstruct Earth’s history over geologic time, geologists rely on several big ideas. One of these is uniformitarianism, the idea that the same geologic processes we can observe today have operated since the beginning of Earth’s history. This means that the same cycles and forces that shape the planet today can help us understand how it has changed since its earliest history. In other words, the present is the key to the past.

11.1. The Timescale for Geologic Change. The Geologic time scale is the history of Earth from its formation about 4.5−4.7 billion years ago to the present. The time scale is divided into eons (longest duration in years), eras, periods, and epochs (shortest duration in years). One important marker is the Cambrian period that represents the beginning of life on a widespread scale on the Earth. The Cenozoic era is the most recent, and nearly all the landscape features visible today were formed within that era.

Suggestion: It may be obvious, but the class should be told that there was life on Earth prior to the Cambrian period (such as bacteria and other organisms), but that the fossil evidence starts with marine trilobites in the early Cambrian.

11.1.2 Forces of Geologic Change. Earth’s surface is constantly changing as old crust is broken down and new crust is formed. Volcanic and tectonic activity brings fresh rock to the planet’s surface. We call these internal or endogenic processes, because they work from within Earth. External or exogenic processes, such as weathering by wind and water, work at Earth’s surface. They lower continental surfaces by removing and transporting mineral matter through the action of running water, waves and currents, glacial ice, and wind.

11.2 The Structure of Earth: Scientists gain knowledge about Earth’s interior from a variety of observations. Active lava flows from volcanoes show us what lies below Earth’s crust. We can study exposed rock layers in road cuts and canyons to learn how sediments and other rock formations are laid down over tens of thousands, even millions, of years to create a record of past climates and Earth-forming events. With deep drilling, we can study the composition of the Earth near the surface. To explore Earth at even deeper levels, we can observe the paths of earthquake waves traveling through its center. A combination of field sampling and laboratory testing, in conjunction with scientific publications and vibrant discussions among Earth scientists, has helped to reveal the hidden world of our planet’s structure.

11.2.1 The Core. Earth’s central core is about 3500 km (about 2200 mi) in radius and is very hot—somewhere between 3000°C and 5000°C (about 5400°F to 9000°F). We know from measurements of earthquake waves passing through Earth that the core has two distinct layers. The outer core is liquid, as demonstrated by the fact that energy waves suddenly change behavior when they reach this boundary. In contrast, the inner core is solid and made mostly of iron, with some nickel. The inner core remains solid despite the high temperatures because of the extreme pressure of all Earth materials surrounding it.

11.2.2 The Mantle. The core is surrounded by the mantle, a shell about 2900 km (about 1800 mi) thick, made of mafic (a word formed from “ma,” for magnesium-bearing, and “fic,” from ferric, or iron-bearing) silicate minerals. Mantle temperatures range from about 2800°C (about 5100°F) near the core to about 1800°C (about 3300°F) near the crust. The mantle is the largest of Earth’s layers, making up more than 80 percent of the planet’s total volume.

11.2.3 The Crust and Lithosphere. The thin, outermost layer of our planet is the crust.

Earth’s crust is separated from the mantle by the boundary called the Moho (short for Mohorovičić discontinuity). At the Moho, seismic waves indicate that a sudden change in the density of materials occurs. The crust, composed of varied rocks and minerals, ranges from about 7 to 40 km (about 4 to 25 mi) thick and contains the continents and ocean basins. It is the source of soil on the lands, salts of the sea, gases of the atmosphere, and all the water of the oceans, atmosphere, and lands. The lithosphere is the solid, brittle outermost layer of Earth. It includes the crust and the cooler, brittle upper part of the mantle. The asthenosphere, which lies below the lithosphere, is plastic.

11.3 Earth Materials and the Cycle of Rock Change: The most abundant elements in Earth’s crust are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. They exist in a variety of rock combinations and are formed through physical and chemical processes, both endogenic and exogenic. Oxygen, the most abundant element, readily combines with many of these elements in the form of oxides. Crustal elements can form chemical compounds that we recognize as minerals. Rocks are composed of minerals, naturally occurring, inorganic substances.The three classes of rocks are igneous, sedimentary, and metamorphic.

11.3.1 Igneous Rocks. Igneous rocks, are formed when molten material, or magma, solidifies. The magma moves upward from pockets a few kilometers below the Earth’s surface through fractures in older solid rock. There the magma cools, forming rocks of mineral crystals. Intrusive igneous rocks cool slowly below Earth’s surface and develop visible mineral crystals. Extrusive igneous rocks cool rapidly on the land surface or ocean bottom and show microscopic crystals.

Suggestion: John Wiley, the publisher of this text, has its main office in Hoboken, NJ, close to the Hudson River and near the Holland Tunnel. The Palisades sill is an igneous intrusion formation dating from the early Jurassic period of about 200 million years ago. It is located just a short distance to the west of the Wiley office and runs along the western bank of the Hudson (opposite Manhattan) from west‐central Staten Island through Hoboken and ends 40 miles away in Haverstraw, NY. The Palisades are a prominent landmark in the area; as you cross between

New Jersey and New York via the George Washington Bridge you will have an opportunity to see it while you are paying the toll.

11.3.2 Sediments and Sedimentary Rocks. Sedimentary rocks are made from layers of mineral particles found in other rocks (igneous, sedimentary, and metamorphic) that have been released by weathering. They also include rocks made from newly formed organic matter, both plant biomass and invertebrates Most inorganic minerals in sedimentary rocks are from igneous rocks. Sedimentary rocks are composed of sediment, which may be clastic, chemically precipitated, or organic. Layers of sediment are termed strata.

Clastic sedimentary rocks are formed when sediments are compressed and cemented.

Sandstone and shale are common examples. Limestone is formed by chemical precipitation in a marine environment. Hydrocarbons in sedimentary rocks include coal, petroleum, natural gas, and peat. These mineral fuels power modern industrial society.

11.3.3 Metamorphic Rocks. Metamorphic rocks are formed from preexisting rocks by intense heat and pressure, which alter rock structure and chemical composition. Shale is transformed into slate or schist, sandstone into quartzite, limestone into marble, and igneous rocks into gneiss.

Suggestion: An easy thing for the class to understand is that metamorphosed rocks are generally much harder than their previous condition as igneous and sedimentary rocks. For example, no one would ever think of putting shale plates (a relatively weak sedimentary rock) on their roof as compared to slate (a much harder metamorphic rock) that could last for many decades.

11.3.4 The Cycle of Rock Change. The cycle of rock change recycles crustal minerals over many millions of years. This cycle, operating over the geologic timescale, involves various physical, chemical, and biological processes that create, transform, and recycle Earth’s crust. In the surface environment, rocks weather into sediment. In the deep environment, heat and pressure transform sediment into rock that is eventually exposed at the surface.

11.4 Global Topography: The most obvious topographic patterns on Earth’s surface are its relief features—mountain chains, midoceanic ridges, high plateaus, and ocean trenches.

Modern technology allows scientists to map much of Earth’s topography through remote sensing.

11.4.1 Relief Features of the Continents. The two basic subdivisions of continental masses are active belts of mountain making and inactive regions of old, stable rock.

Mountains are built by volcanism and tectonic activity. Inactive continental regions of stable rocks include continental shields and ancient mountain roots. Continental shields are low-lying areas of old igneous and metamorphic rock.

11.4.2 Relief Features of the Ocean Basins. Oceans make up 71% of Earth ’s surface.

Relief features of oceans are quite different from those of the continents. The ocean basins include a midoceanic ridge with a central axial rift, where crust is being pulled apart. Much of the oceanic crust is less than 60 million years old, compared to the great

bulk of the continental crust, which is over 1 billion years old. The youth of the oceanic crust is quite remarkable.

11.4.3 Configuration of the Continents. Alfred Wegener proposed that today’s continents had broken apart from a single supercontinent named Pangaea. Although many doubted his ideas, he was eventually proven right. Ocean basins include a midoceanic ridge with a central axial rift where crust is being pulled apart. The continents are moving today, just as they have in the past. Data from orbiting satellites show that rates of separation or convergence of two plates are on the order of 5 to 10 cm (about 2 to 4 in.) per year, or 50 to 100 km (about 30 to 60 mi) per million years. At that rate, global geography must have been very different in past geologic eras from what it is today.

11.5 Plate Tectonics: As described earlier in this chapter, Earth’s lithosphere is fractured into more than 50 separate tectonic plates, ranging from very large to very small. The motions of lithospheric plates are responsible for shaping our planet, from the tops of mountains to the trenches of the sea bottom. Over geologic time, changes in the configuration of oceans and continents produced by the movement of lithospheric plates have altered Earth’s climates and influenced the distribution and abundance of plants and animals across the planet. The body of knowledge about lithospheric plates and their motions is referred to as plate tectonics.

11.5.1 Extension and Compression. Tectonic processes include extension and compression. Extension, or rifting, occurs when the lithosphere is pulled apart.

Compression occurs when plates are pushed together. Extension causes fracturing and faulting of the crust, while compression produces folds and overthrust faults.

11.5.2 Plate Boundaries. At a spreading boundary, crust is being pulled apart. At a converging boundary, one plate is subducted beneath another. At a transform boundary, two plates glide past each other.

11.5.3 Continental Rupture and New Ocean Basins. Continental rupture begins with the formation of a rift valley and tilted block mountains. Ocean soon invades the rift. As the continental crust recedes, oceanic crust fills the gap.

Suggestion: The Red Sea is an excellent example of a continental rupture and three spreading boundaries that were created by movement of the Arabian plate away from the African plate.

Mention to the class that this will eventually become an ocean over a very long period of time measured in millions of years.

11.5.5 Island Arcs and Collision of Oceanic Lithospheric Plates. At a convergent boundary where plates of oceanic lithosphere collide, an arc of volcanic islands rises just beyond the subduction zone. As the process continues, seafloor sediment piles up in the trench, taking shape as an accretionary wedge of sediments. Meanwhile, the continuing rise of magma fortifies the island arc from below, increasing the height and width of the volcanic mass.

11.5.6 Arc-Continent Collision. Eventually, the island arc can collide with a passive continental margin, producing an arc-continent collision. The mass of collided rocks is called an orogen, and the process of its formation is described as an orogeny.

11.5.7 Contuinent-Continent Collision. When two continental lithospheric plates collide in an orogeny, continental rocks are crumpled and overthrust. The plates become joined in a continental suture.

11.5.8 The Wilson Cycle and Supercontinents. Ocean basins open and close in the Wilson cycle, which describes how continents split and are reunited. As many as six to 10 supercontinents have formed throughout Earth’s history.

11.5.9 The Power Source for Plate Movements. Lithospheric plates are huge, so it must take enormous power to drive their motion. This power comes from radiogenic heating caused by radioactive decay of unstable isotopes that occur naturally in the rock beneath the continents. We don’t know exactly how radiogenic heating sets plates in motion, but one theory is that they are generated by convection currents in hot, plastic mantle rock.

Since hotter rock is less dense than cooler rock, unequal heating could produce streams of upwelling mantle rocks that rise steadily beneath spreading plate boundaries.

Suggestion: It would be both interesting and useful to mention to the class that Europe and North America are slowly but surely moving apart from one another. The rate of movement is of course extremely slow in human terms, but the end result is an increase in the distance. It is always worthwhile to discuss the enormous difference between our sense of human time and geologic time.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (800‐255‐8629). Plate Tectonics. DVD, # DV 3626.

Crystal Productions, Glenview, IL (800‐255‐8629). The Birth of a Theory; Plate Dynamics. VHS #VC‐3520, 30 minutes each.

Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126). Plate Tectonics. DVD # PCT 347‐22‐K; VHS #PCT 347‐22‐A, approx. 22 minutes, 2006.

Insight Media, New York, NY (800‐233‐9910). Plate Tectonics. DVD # 8AS‐2495, 25 minutes, 2006.

Insight Media, New York, NY (800‐233‐9910). Plate Tectonics. DVD # 79‐AS‐1116, 15 minutes, 2007.

Select Reference Books: Anderson, Don L. New Theory of the Earth. New York, NY: Cambridge University Press, 2007.

Dixon, Timothy H. and J. Casey Moore. The Seismogenic Zone of Subduction Thrust Faults.

Irvington, NY: Columbia University Press, 2007.

Huggert, Richard J. The Natural History of Earth. New York, NY: Routlege, 2007.

Kearey, Philip, Keith A. Klepsis, and Frederick J. Vine. Global Tectonics. Wiley, 3rd ed., 2009.

Rollinson, Hugh R. Early Earth Systems: A Geochemical Approach. Wiley, 2007.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 12: Tectonic and Volcanic Landforms Chapter Objectives: Upon completion of this chapter the student will be able to: 1) Describe landforms produced by tectonic activity.

2) Explain the relationship between earthquakes and plate tectonics.

3) Describe landforms produced by volcanic activity.

Volcanoes and earthquakes are usually of great interest to students because of their dramatic qualities. Real‐time and near‐real‐time websites provide a compelling case for tectonic landforms being part of continuous, dymanic Earth processes. Geologic structures are often entirely new to many students and can be difficult to visualize.

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Volcanoes: There are numerous websites concerning volcanoes, many of which have extensive collections of images.

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Near‐real‐time volcano sites: Students often imagine that volcanoes erupt for a very short period of time; however, volcanic activity can occur over very long periods with characteristics that are specific to each volcano. Assign your students to choose one volcano that is currently active and describe its behavior. The Weekly Volcanic Activity Report is also a useful source of current information.

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Urban volcanoes: Volcanoes are often perceived as remote, yet there are many situated close to major population centers. One example is Vesuvius, detailed in the Nova presentation The Deadly Shadow of Vesuvius.

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Field trips: There are also a number of virtual field trips, such as the Volcano Expedition from the Field in Costa Rica site. Assign students to participate in one of the virtual field trips and write an account of their expedition.

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Earthquakes: Students often do not realize that earthquake activity is a continual process.

The World Wide Earthquake Locator provides continuous updates on earthquake activity.

Follow these sites during class and often you will see earthquakes added.

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Visualizing Rock Structures: A wide range of examples and illustrations helps students differentiate various structural concepts. Use sites such as the Landscape Evolution site to provide a broad array of photographs. Challenge students to find local examples of different types of rock structures.

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Virtual Field Trips: – There are also a number of virtual field trips such as the Oneonta to the Hudson River site and the New Mexico site. Assign your students to write an account of a virtual field trip of their choice, while noting the features that they have learned about from lectures and textbook readings.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Weekly Volcanic Activity Report, Smithsonian USGS. A weekly global report listing currently erupting volcanoes and the details of their activity. http://www.volcano.si.edu/gvp/reports/usgs/index.cfm Nova‐The Deadly Shadow of Vesuvius. A well-illustrated site originally produced for the TV series Nova on one of the world’s most famous volcanoes. http://www.pbs.org/wgbh/nova/vesuvius/ Nova—Into the Abyss: Volcanoes of the Deep. Another well-illustrated site produced for Nova on submarine volcanoes, such as those associated with the Mid‐Atlantic Ridge. http://www.pbs.org/wgbh/nova/abyss/ Hawaii Volcanoes National Park. A site describing the many features of the Hawaii Volcanoes National Park, including wildlife and plant life as well as geology. http://www.nps.gov/havo/ Hawaiian Volcano Observatory. A description of the nature and activities of a well-known active volcano observatory. http://hvo.wr.usgs.gov/ Volcano Expedition from the Field in Costa Rica. A multimedia site describing many features associated with volcanoes in Costa Rica. http://www.sio.ucsd.edu/volcano/ Volcanoes.com. A general site covering many aspects of volcanoes for general interest. http://www.volcanoes.com/ The World-Wide Earthquake Locator. Another constantly updated list that shows earthquakes, includes a mapping program to help locate them. http://www.geo.ed.ac.uk/quakes/quakes.html The Earthquake Experience. A simple simulation of what the shaking of an earthquake might look like. http://www.iris.edu/gifs/animations/faults.htm

Memento Mori. Displays streaming seismographic data measured continuously from a site near the Hayward Fault above the University of California, Berkeley. http://memento.ieor.berkeley.edu/memento.html Tsunami. An in‐depth site describing many characteristics of tsunamis, including historic case studies and methods used to predict and track them. http://www.geophys.washington.edu/tsunami/welcome.html Virtual Tour of New Mexico Geology. This site includes information about Ship Rock, an eroded volcanic neck with radial dikes. http://geoinfo.nmt.edu/tour/home.html Structural Geology on the Web. Includes links to a wide variety of images, online courses, and fieldtrips. http://www.structural-geology.org/ Image Gallery Landform Wing from Geology at About.com. An array of images to illustrate many geologic structures. http://geology.about.com/library/bl/images/bllandformindex.htm The Idaho Batholith. Maps and explanations of this important igneous intrusion. http://imnh.isu.edu/digitalatlas/geo/bathlith/bathdex.htm Geology of the Black Hills of South Dakota. Includes information on the Black Hills Dome. http://homestake.sdsmt.edu/Core/Core%20Archive.pdf

Chapter Lecture: 12.1 Tectonic Landforms: Plate tectonics provides a convenient framework for our study of landforms, which begins with this chapter. The motion of lithospheric plates warps, folds, and breaks apart rock layers in the processes occurring at spreading, converging, and transform boundaries. This creates rock structures such as folds, faults, and thrust sheets. Although these structures are often formed deep within Earth, erosion over many millions of years can remove great thicknesses of overlying rocks to reveal them at the surface. The result is a group of landforms that depend on both rock structure and the way particular rock layers and rock bodies—weak or strong—respond to erosion.

12.1.1 Rock Structures of Converging Boundaries. Upfolds are anticlines and downfolds are synclines. Fold belts create a ridge-and-valley landscape of alternating ridges of resistant rock and valleys of weak rock. Slate and marble are weak metamorphic rocks that underlie valleys. Schist, gneiss, and quartzite are more resistant and underlie uplands and ridges. Erosion of monoclines and domes produces hogback ridges, flatirons and, in the case of domes, circular valleys.

Suggestion: Mention to students that the next time they travel on an interstate highway, they might pay particular attention to rock exposures in roadcuts to look for any faulting or folding.

12.1.2 Plate Interiors. Far from spreading and converging boundaries, the centers of continental lithospheric plates exhibit rock structures, sometimes quite ancient, that also produce distinctive landforms. Where the stable continental interior of a lithospheric plate has been stripped of sedimentary cover by long erosion, shield rocks are exposed.

These rocks include highly metamorphosed rocks and crystalline igneous rocks of ancient origin, which may be related to supercontinent cycles occurring long, long ago.

Many of the igneous rocks are huge batholiths of felsic rock that melted deep within Earth. Because batholiths are typically composed of resistant rock, they are eroded into hilly or mountainous uplands. Isolated mountains or hills revealed by erosion of weaker surrounding rock are called monadnocks.

12.1.3 Coastal Plains. A large lithospheric plate may include a passive margin where continental crust meets oceanic crust. In this case, the coastline receives large quantities of sediments from the continental interior that are deposited in shallow waters just offshore. If the sea level falls or the passive margin rises, a gently sloping coastal plain of recent sediments emerges. Beds of sand and clay alternate. The clay forms lowlands, whereas the sandy ridges form lines of low hills called cuestas.

12.1.4 Faults and Fault Landforms. In a fault, rocks break apart and move in different directions. The four main types of faults are normal, strike-slip or transcurrent, reverse, and overthrust. Parallel normal faults are caused by extension and produce downdropped blocks (grabens) and upthrown blocks (horsts).

12.2 Earthquakes: An earthquake is a seismic wave motion transmitted through Earth. It is triggered by sudden slippage along a fault.

12.2.1 Epicenter and Focus. In an earthquake, the location where the fault slipped, called the focus, can be near the surface or deep underground. The depth of an earthquake’s focus in part determines the intensity of shaking felt on the ground.

Shallow-focus earthquakes are likely to cause more damage than deep-focus earthquakes.

The point on Earth’s surface directly above the focus is called the epicenter of the earthquake. Earthquakes generate Primary (P-) and Secondary (S-) waves that radiate outward from the earthquake focus.

Suggestion: Ask your students if any of them have ever experienced an earthquake. If so, have them discuss it with the class.

12.2.2 Magnitude. The amount of energy released by an earthquake, called its magnitude, can be measured by the amplitude of the seismic waves produced. The Richter Scale is used to measure the energy released by earthquakes.

12.2.3 Tectonic Environments of Earthquakes. Earthquakes occur frequently at spreading and converging boundaries of lithospheric plates. Transcurrent faults on transform boundaries are also common earthquake sites.

12.2.4 Subduction-Zone Earthquakes. Intense seismic activity occurs along convergent lithospheric plate boundaries where oceanic plates are undergoing subduction. This mechanism is responsible for the greatest earthquakes, including those in Japan, Alaska, North America, Central America, and Chile, and other narrow zones close to the trenches and volcanic arcs of the Pacific Ocean Basin. Subduction-zone earthquakes can also produce tsunamis. Tsunamis occur when the subducted plate suddenly snaps back during an earthquake. It displaces a large volume of water, generating long-wavelength water waves that can travel hundreds of miles across the open ocean. s these waves approach the shore, they slow and build into a wall of water that can be meters high.

Suggestion: Mention to the class that those students who live on the east coast of the U.S., or vacation at certain large inland lakes in mountainous areas, can also experience tsunamis, albeit at lower probabilities. For example, any movement or sliding along the outer continental shelf of the U.S. could generate a tsunami that could spread to the east coast. Another example would be Lake Tahoe in the Sierras, which is shared by California and Nevada. It is a lake of great depth (1,645 ft, 501 m) that is underlain by faults that could move and generate tsunami with heights reaching 33 ft (10 m).

12.2.5 Transform Boundaries. Strike-slip faults on transform boundaries that cut through the continental lithosphere cause moderate to strong earthquakes. The San Andreas Fault, which runs about 1000 km (about 600 mi) from the Gulf of California to Cape Mendocino, is an active strike-slip fault. The San Andreas Fault and related faults in the southern and central areas of California are potential sources of great earthquakes occurring in densely populated regions. Some earthquakes (isolated earthquakes) occur far from active plate boundaries, for unexplained reasons. Although scientists understand the role that faults play in earthquake risk, they don’t always know where fault systems are located. Some, called blind faults, are not apparent on Earth’s surface. Seismographic research can help scientists identify blind faults, but they are hampered in this effort by political unrest in some areas and the geographic inaccessibility of others, which may contain fault systems that have never been mapped.

12.2.6 Volcanic Activity and Landforms: Like earthquakes and plate tectonics, volcanic activity creates initial landforms. Underground molten mineral matter, called magma, is extruded through constricted vents and fissures in Earth’s surface. When the magma reaches the surface as lava, it cools and hardens, building the landform familiarly known as a volcano, which is typically conical or dome-shaped.

12.6.1 Tectonic Environments of Volcanoes. Volcanic activity is frequent along subduction boundaries, which account for the Ring of Fire around the Pacific Rim. Mid-ocean spreading centers and hotspots also provoke volcanic activity.

12.6.2 Volcanic Eruptions. Volcanic eruptions can have extreme environmental impacts.

Flows of hot gas, showers of ash, cinders, and rocks, violent earthquakes, and accompanying tsunamis can cause great loss of life.

12.6.3 Types of Volcanoes. The shape, size, and explosiveness of a volcano depend

on the type of magma involved. Magma comes from two main types of igneous rocks: felsic and mafic. There are three principal types of volcanoes: 1) Stratovolcanoes, 2) Shield Volcanoes, and 3) Cinder Cones. Stratovolcanoes are tall, steep cones built of layers of felsic lava and volcanic ash. Felsic magma can contain gases under high pressure, so felsic eruptions are often explosive. In contrast to the explosive eruptions of felsic stratovolcanoes, eruptions of shield volcanoes are usually quiet. Basaltic lava flows smoothly over long distances and spreads out in thin layers. Most of the lava flows from fissures (long, gaping cracks) on the flanks of the volcano. Shield volcanoes are marked by erosion features that are quite different from those of stratovolcanoes. Mauna Loa in Hawaii is a distinctive shield volcano. Cinder cones are small volcanoes that form when frothy magma is ejected under high pressure from a narrow vent, producing tephra. The rain of tephra accumulates around the vent to form a roughly circular hill with a central crater. Cinder cones rarely grow more than a few hundred meters high. An exceptionally fine example of a cinder cone is Wizard Island, Oregon, which was built on the floor of Crater Lake long after the caldera was formed.

12.6.4 Hot Springs, Geysers, and Geothermal Power. Where hot rock material is near Earth’s surface it can heat nearby groundwater to high temperatures. When the groundwater reaches the surface, it provides hot springs at temperatures not far below the boiling point of water. At some places, jet-like emissions of steam and hot water occur at intervals from small vents, producing geysers. The water that emerges from hot springs and geysers is largely groundwater that has been heated in contact with hot rock, and is thus recycled surface water. Little, if any, is water that was originally held in rising bodies of magma. The heat from masses of lava close to the surface in areas of hot springs and geysers provides a source of energy for electric power generation. Here, the groundwater has been heated intensely, but because of the overlying pressure of the rock, it remains in a liquid state. To generate power, wells are drilled to tap the hot, pressurized water, which flashes into steam when it is released at the surface. The steam then drives turbines that generate electric power

Select Audiovisual Aids: Crystal Productions, Glenview, IL (800‐255‐8629). Lava Flows and Lava Tubes. DVD #DV‐3628, VHS VC 3629, 75 minutes.

Crystal Productions, Glenview, IL (800‐255‐8629). Volcanism; Intrusive Igneous Rocks. #VC ‐ 3524, 30 minutes each.

Films for the Humanities and Sciences, Hamilton, NJ (800‐329‐6687). Geothermal Energy: Tapping the Earth’s Heat. DVD PCT #344‐28‐KS; VHS PCT #398‐28‐A, 15 minutes, 2000.

Films for the Humanities and Sciences, Hamilton, NJ (800‐329‐6687). Inside an Earthquake.

DVD PCT #330‐91‐KS; VHS PCT #330‐91‐A; 26 minutes, 2000.

Insight Media, New York, NY (800‐233‐9910). Volcanoes of the United States. DVD # 79‐AM‐591, 24 minutes, 2004.

Select Reference Books: Fortey, Richard. Earth: An Intimate History. New York, NY: Alfred A. Knopf, 2004.

Geschwind, Carl‐Henry. California Earthquakes: Science, Risk, and the Politics of Hazard Mitigation. Baltimore, MD: Johns Hopkins University Press, 2001.

Lamb, Simon and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998.

Parfitt, Liz and Lionel Wilson. Fundamentals of Physical Vulcanology. Wiley, 2007.

Rhodes, Frank H.T., Richard O. Stone, and Bruce D. Stone (eds.) Language of the Earth: A Literary Anthology. Wiley, 2008, 2nd ed.

Sharma, Vijay K. Process Geomorpholgy. Boca Raton, FL: CRC Press, 2010.

Thompson, Graham R. and Jonathan Turk. Earth for Earth Science and the Environment.

Brooks/Cole, 2011.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 13: Weathering and Mass Wasting Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe effects of physical and chemical weathering processes.

2) Describe slopes and identify the processes that form them.

3) Explain how gravitational forces produce mass wasting.

Weathering and mass wasting are straight forward topics that require students to understand terminology and build upon their knowledge of rock types. Use the following sites to show photographs of various phenomena, as well as significant case studies.

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Types of weathering and erosion: A wide range of examples and illustrations helps students differentiate the various concepts. Use sites such as the Images of Weathering site from Duke University to provide a broad array of photographs. Challenge students to find local examples of different forms of erosion (the Gravestone Weathering site may provide some inspiration).

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Landslides and earthflows: Students often do not realize that catastrophic events are not restricted to weather and tectonics and may consider mass movements to be entirely innocuous. Use the sites that focus on case studies to describe and explain the impact of these hazards.

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Finding out more: Many of these websites also provide additional information on some of the examples in the text. Point your students toward these to find out more details perhaps for an essay project.

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Periglacial landscapes: Many students are completely unfamiliar with periglacial landscapes, and although they may be aware of permafrost, often do not know about the implications for the surface. Use the web sites, such as the Periglacial Processes in the Yukon Territory site, to show examples and how landforms occur together.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Images of Weathering. Many examples with photographs of different types of weathering from Duke University. http://www.env.duke.edu/eos/geo41/wea.htm Gravestone Weathering. An interesting application from the UK on the analysis of weathering of gravestones. http://www.envf.port.ac.uk/geo/inkpenr/graveweb/gravestone.htm Turtle Mountain Landslide. A detailed account of an important landslide covered in the text. http://www3.sympatico.ca/goweezer/canada/frank.htm Hebgen Lake Landslide. A general account of an important landslide covered in the text. http://www.seis.utah.edu/NEHRP_HTM/1959hebg/c1959he1.htm Periglacial Processes in the Yukon Territory. An introduction to the different types of periglacial landforms evident in the Yukon. http://sis.agr.gc.ca/cansis/taxa/landscape/ground/yukon.html Chapter Lecture: 13.1 Weathering: The two basic forms of weathering are: physical weathering, whereby rocks are fractured and broken apart, and chemical weathering by which the minerals in the rock are chemically altered into softer, more soluble forms. The end result of these processes is the formation of a surface layer of weathered rock particles called regolith that forms over the subsurface unaltered solid rock.

13.1.1 Physical Weathering. Physical weathering, also known as mechanical weathering, fractures rock into smaller pieces, without chemical alteration of the minerals. There are five principal forms of mechanical weathering. Frost action is a very important physical weathering process that is quite common in cold climates. Under the proper conditions, when water freezes in bedrock joints and bedding planes, it can expand and split rocks apart. Salt‐crystal growth is another physical weathering process similar to frost-cracking.

In arid climates, slow evaporation of groundwater from outcropping sandstone surfaces causes the growth of salt crystals. Crystal growth breaks the rock apart, grain by grain, producing niches, shallow caves, and rock arches. Thermal action cracks rocks when temperature changes cause minerals to expand and contract at different rates. In exfoliation, rock layers crack as the pressure of overlying rocks is reduced by erosion.

Biological action, such as the active growth of plant roots, can exert pressure strong enough to wedge joint blocks apart and break up rock.

13.1.2 Chemical Weathering. In chemical weathering, the minerals that make up rocks are chemically altered or dissolved. The end products are often softer and bulkier forms that are more susceptible to erosion and mass movement. There are three principal forms of chemical weathering. In hydrolysis, minerals react chemically with water. For example, the mineral feldspar, a component of granite, reacts to form a soft clay mineral and silica residue that are readily washed or blown away. Through this process, granite can weather to produce interesting boulder and pinnacle forms. Thermal action or other weathering processes can then further degrade the rock. In oxidation, oxygen and water react with metallic elements in minerals, making the minerals unstable and causing the rock to degrade in strength and eventually crumble into smaller particles. Because chemical oxidation produces distinctive colors in rock, geologists can use the shades of ancient rock layers to identify periods when less oxygen existed in the atmosphere. Acid action is the third chemical weathering process. Carbonic acid is a weak acid formed when carbon dioxide dissolves in water. Found in rainwater, soil water, and stream water, carbonic acid slowly dissolves some types of minerals, in a process called carbonation. Carbonate bedrocks, such as limestone and marble, are particularly susceptible to carbonation, and produce many interesting surface forms. Carbonic acid in groundwater dissolves limestone, creating underground caverns and distinctive landscapes that develop when these caverns collapse. Soil acids, which are formed as microorganisms digest organic matter, rapidly dissolve basaltic lava in wet, low-latitude climates.

Suggestion: If anyone in the class is not sure about the dissolving power of carbonic acid, a visit to a local cemetery, particularly one that contains the oldest gravestones in the area, will clearly show the ravages of decomposition on marble or limestone gravestones. Many of the names on these gravestones are no longer legible.

13.2 Slopes and Slope Processes: Once rock fragments have been loosened from parent rock through physical or chemical weathering they are subjected to gravity, running water, waves, wind, and the flow of glacial ice. We concentrate here on how Earth materials are moved by gravity and address the other effects in the following chapters.

13.2.1 Slopes. Slopes are mantled with regolith, which accumulates at the foot of slopes as colluvium. Regolith that is transported by moving water is termed alluvium.

13.2.2 Slope Stability. Counterbalancing the downward force of gravity are the resisting force of the cohesiveness of the rock material and the internal friction holding it in place.

Slopes maintain a condition of dynamic equilibrium, which means that material on a slope stays in place until one or another counterbalancing factor causes the slope system to readjust into a new state of equilibrium. Where and when materials on a slope respond to the constant forces of gravity by moving downhill are determined by the angle of the slope, the slope material, and the combination of weathering processes 13.3 Mass Wasting: Mass wasting is the spontaneous movement of soil, regolith, and rock under the force of gravity. There are many types of mass wasting, depending on the speed of the motion and the amount of water involved.

13.3.1 Creep. The slowest form of mass wasting is soil creep. On almost every soilcovered slope, soil and regolith are slowly and imperceptibly moving downhill. This common movement is apparent in older neighborhoods and farms, where one can see fence posts and retaining walls leaning in a downhill direction. The process is triggered when soil and regolith are disturbed by alternate drying and wetting, growth of ice needles and lenses, heating and cooling, trampling and burrowing by animals, and shaking by earthquakes. Gravity pulls on every such rearrangement, and the particles very gradually work their way downslope.

Suggestion: Ask students if they know of any specific examples of creep.

13.3.2 Rockfalls and Talus. The most visible form of mass wasting we are likely to encounter is a rockfall in which rocks fall down steep slopes or cliff sides, often bringing soil or regolith along for the ride. Loose fallen rocks called talus, or scree, can collect at the bottom of a slope or cliff in a cone-shaped pile. Talus slopes become picturesque boundaries along the base of many majestic mountain escarpments.

13.3.3 Slides. In a landslide, a large mass of rock suddenly moves from a steep mountain slope to the valley below. Landslides are triggered by earthquakes or rock failures rather than heavy rains.

13.3.4 Flows. Mass wasting of Earth materials with high water content results in flows.

Flows occur when precipitation or snowmelt is greater than absorption into the underlying sediment or rock. In deserts, for example, thunderstorms produce rain much faster than it can be absorbed by the soil. After a wildfire, hillsides are similarly vulnerable to flows. There are two principal types of flows. An earthflow is a mass of water-saturated soil that moves slowly downhill. Earthflows can block highways and railroads and severely damage or destroy buildings. Mudflows are rapid events in which water, sediment, and debris cascade down slopes and valleys to lower elevations. They are produced by very heavy rainfall or snowmelt caused by volcanic activity.

13.3.5 Induced Mass Wasting. Human activities can induce mass wasting by piling up unstable materials or undercutting slopes or rock masses. Cutting and filling to extract mineral resources is termed scarification.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (1‐800‐255‐8629). Weathering and Erosion. DVD DV‐3198, 22 minutes.

Crystal Productions, Glenview, IL (1‐800‐255‐8629). Weathering and Erosion. DVD DV‐3181; VHS VC‐3653, 23 minutes.

Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126). Arctic Rush: Staking a Claim in the Earth’s Uncertain Future. DVD PCT 400‐74‐KS; VHS PCT 400‐74‐A, 47 minutes, 2006.

Insight Media, New York, NY (800‐233‐9910). Weathering and Erosion. DVD #8AS‐2211, 24 minutes, 2000.

Insight Media, New York, NY (800‐233‐9910). When the Earth Moves. DVD #79AR‐1262, 27 minutes.

Select Reference Books: Arctic Climate Assessment Committee. Ogdensburg, New York: Renouf Publishing Co., 2005.

Bryant, Edward. Natural Hazards. New York, NY: Cambridge University Press, 2005.

Filler, Dennis M., Ian Snape, and David L. Barnes (eds.). Bioremediation of Petroleum Hydrocarbons in Cold Regions. New York, NY: Cambridge University Press, 2008.

Glade, Thomas, Malcolm G. Anderson, and Michael J. Crozier (eds.). Lansdscape Hazard and Risk. Wiley, 2005.

Smith, Keith and David N. Petley. Environmental Hazards: Assessing Risk and Reducing Disaster. NewYork, NY: Routledge, 5th ed., 2008.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 14: Fresh Water of the Continents Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe the flow of fresh water in the hydrologic cycle.

2) Describe the movement of ground water.

3) Describe how groundwater is used and identify problems of groundwater management.

4) Discuss the movement of surface water and behavior of streams.

5) Identify the causes and effects of floods.

6) Describe the characteristics of lakes.

7) Discuss our reliance on water as a natural resource.

Water and its movement across the land surface is often mistakenly assumed to be entirely encompassed by rivers and lakes. Caves and cave features provide eye‐catching examples of the role of groundwater, while other sites emphasize the need to protect groundwater resources.

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Caves: The broad range of cave web sites with their stunning imagery can rapidly develop student interest. As many have great cultural significance, such as the Lascaux Caves, these can also be useful in generating interest in arts and humanities students.

Assign students to review one of the cave sites and identify the cave features covered in the text.

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Groundwater: The increasing importance of groundwater as a resource for drinking water and irrigation can be used to inspire students to understand the problems of depletion and pollution. Assign students to investigate the groundwater of the local region and identify potential threats.

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Floods: In general, people often underestimate the hazards associated with flooding. Use websites, such as the Fargo site, to demonstrate the impact of such events. Assign students to examine the Significant Floods in the US site and identify the most recent, the most damaging, and the closest flood events.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region. (Functioning 12/3/12)

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Mammoth Cave National Park. A virtual tour of the most extensive cave network in North America. http://www.nps.gov/maca/planyourvisit/gocavetours.htm USGS Groundwater Information Pages. An extensive introduction to the processes associated with groundwater. http://water.usgs.gov/ogw/ Groundwater Primer (EPA). Download this self-extracting file to see a general introduction on groundwater. http://water.epa.gov/drink/ The Caves of Lascaux. A tour of caves in northern France, famous for their exceptional examples of early cave paintings. http://www.culture.fr/culture/arcnat/lascaux/en/ Colossal Cave Mountain Park, Tucson, Arizona. A virtual tour of a cave in the Sonoran desert. http://www.colossalcave.com/cavetour.html Water Conflict Chronology. A history of conflicts worldwide over water supplies. http://www.worldwater.org/conflictchronology.html Significant Floods in the US in the Twentieth Century. Provides historical accounts and assesses damage. http://ks.water.usgs.gov/Kansas/pubs/fact%2Dsheets/fs.024%2D00.html Nova—Flood! A web site created for the TV series Nova that examines the causes and effects of serious floods. http://www.pbs.org/wgbh/nova/flood/

Chapter Lecture: 14.1 Freshwater and The Hydrologic Cycle: Water is essential to life. Nearly all organisms, including humans, require constant access to water, or at least a water-rich environment, for survival. The overwhelming majority of the water on Earth (97%) is contained within the oceans and is therefore salty. The remaining 3% is freshwater, of which the largest share (2.1%) is frozen in icecaps (Antarctica and Greenland) and glaciers. Groundwater accounts for about 0.61% of the total freshwater, but half of that is deeper than 0.62 mi (1 km) and is, therefore, relatively unavailable due to pumping expense. Surface water (including lakes) accounts for about 0.02% of the total water on Earth. Surprisingly, the estimated average instantaneous flow of all the streams in the world (including the mighty Amazon) amounts to only 0.0001% of the total water on this planet.

14.1.1 Paths of Precipitation. Precipitation that falls over land either runs off or infiltrates into the soil. As runoff, it flows into streams. As infiltration, it returns to the air through evapotranspiration or percolates downward to become groundwater.

14.2 Groundwater: Groundwater is found in that part of the subsurface water that has totally saturated the pore spaces in bedrock, regolith, and the soil. The water table is found at the top of this saturated zone. The unsaturated zone occurs where the pores are not fully saturated and therefore includes the soil‐water belt. The depth of this unsaturated zone varies from practically zero in swamps and marshes to hundreds of feet (meters) in arid regions.

14.2.1 The Water Table. The water table marks the top of the saturated zone of groundwater. It is highest under hilltops and divides and slopes to intersect the surface at lakes, marshes, and streams.

Suggestion: It would be useful to show the class what a water table map looks like. The contours do much of the explanation and even a simple sketch of a sample area would be quite helpful.

Instructors might want to check with their state geological surveys for such sample maps, as one illustration is worth so many words.

14.2.2 Aquifers. The amount of groundwater that can be held in the saturated zone depends on the porosity of the sediments that make up this layer. If the layer is porous and permeable enough to hold and conduct a usable quantity of water, and if the water can be easily pumped from the material, then it is called an aquifer. A bed of sand or sandstone is often a good aquifer because clean, well sorted sand—such as that found in beaches, dunes, or stream deposits — can hold an amount of groundwater equal to about one-third of its bulk volume. Sandy materials have large pore spaces that allow the water to move through the sediment and be readily pumped from wells. By contrast, layers that are relatively impermeable to groundwater are known as aquicludes. Clay and shale beds are examples of materials that do not conduct water in usable amounts. When an aquifer is sandwiched between two impermeable aquicludes, pressure may force the water to rise to the surface as a self-flowing artesian well. A fault can serve as a natural conduit for groundwater, producing artesian springs.

Suggestion: Mention to the class that the difference between aquifers and aquicludes can range over an order of magnitude, such as some glacial deposits that have very favorable permeability and porosity values compared to some rock formations such as argillite and diabase that are noted for their very low groundwater yields. That is why some forward‐thinking states, counties, and municipalities mandate pump tests for homes or commercial buildings that are not served by public water systems and therefore must rely on local groundwater sources.

14.2.3 Limestone Solution by Groundwater. Limestone solution by groundwater can produce caverns as a consequence of carbonic acid dissolving the rock formation.

Sinkholes are surface depressions of varying size that are associated with areas of cavernous limestone. Regions that are associated with limestone formations, numerous sinkholes, and a paucity of small surface streams form a karst landscape. The term karst comes from the German version of the Slavic term kras or krs that refers to a waterless area. Karst‐like topography is found in Croatia, Cuba, the northern part of Puerto Rico, southern China, and the Mammoth Cave region of Kentucky.

Suggestion: The class should be interested to know that sinkholes are quite abundant in the Orlando, Florida area, close to the Walt Disney World complex. The pattern of circular sinkholes in the area is rather striking, especially when a new one slowly starts to develop. You might also mention to the class that there are truly “disappearing streams” in karst areas. They usually have “Lost River” in their names, and in certain areas, you can actually see a surface stream simply disappear underground and reappear at the surface at a different point in the landscape. In fact, West Virginia has created a Lost River State Park as a tourist attraction.

14.3 Groundwater Use and Management: Groundwater is a substantial source of freshwater for human use; but it is a finite resource that requires precipitation to replenish it. It is also affected by both withdrawal and pollution, both of which can have serious and long-lasting environmental consequences on the supply of freshwater.

14.3.1 Groundwater Withdrawal. Rapid withdrawal of groundwater has had a serious impact on the environment in many places. As water is pumped from a well, the level of water in the well drops. At the same time, the surrounding water table is lowered, in the shape of a downward-pointing cone called the cone of depression. The difference in height between the cone tip and the original water table is known as the drawdown.

Where many wells are in operation, their intersecting cones will lower the water table.

Suggestion: Discuss with the class the problems that Venice, Italy is experiencing, due to excessive groundwater withdrawal for industrial uses. For a closer example, mention the extreme loss of groundwater from the Ogallala Aquifer and its associated problems. This is a good time to emphasize how critically important an abundance of freshwater is to all aspects of life (nutritional, industrial, recreational, etc.).

14.3.2 Subsidence. An important environmental effect of excessive groundwater withdrawal is subsidence, the sinking, of the ground surface.

Suggestion: Discuss severe land subsidence problems in Mexico City to illustrate this topic to your class.

14.3.3 Pollution of Groundwater. Another major environmental problem is the contamination of groundwater by pollutants such as agricultural runoff, industrial waste, and acid rain, among other sources. Sanitary landfills and overland flows can carry pollutants and toxic compounds to the water table, thereby contaminating groundwater.

When aquifers are overpumped in coastal regions, saltwater can contaminate groundwater through a process called saltwater intrusion. Because freshwater is less dense than saltwater, a layer of saltwater from the ocean can lie below a coastal aquifer.

As the aquifer is depleted, the level of saltwater rises and eventually reaches the well from below, making the well unusable.

14.4 Surface Water and Streamflow: Recall from the section discussing the hydrologic cycle that some precipitation filters through the soil to become groundwater and some travels over the surface to streams, which eventually carry the water back to the ocean. We have already seen

what happens to water that infiltrates the soil. In this section we take a closer look at water that travels over the surface in streamflow.

14.4.1 Overland Flow. When soils are saturated or rain falls too quickly to be absorbed into the ground, water travels directly over the surface as overland flow. Where the soil or rock surface is smooth, the flow may be a continuous thin film, called sheet flow. If the ground is rough or pitted, overland flow may be made up of a series of tiny rivulets connecting one water-filled hollow with another.

14.4.2. Drainage Systems. As runoff moves to lower and lower levels and eventually to the sea, it becomes organized into a branched network of stream channels. This network and the sloping ground surfaces next to the channels that contribute overland flow to the streams are together called a drainage system. A drainage basin, or watershed, consists of a branched network of stream channels and adjacent slopes that feed the channels. It is bounded by a drainage divide.

14.5 Flooding: When soil is saturated by snowmelt or precipitation, runoff fills streams and rivers. When the discharge of a river cannot be accommodated within the normal channel, the water spreads over the adjacent ground, causing a flood.

14.5.1 Floodplains. A flood occurs when a river rises over its banks and covers adjacent land, which is called the floodplain. The height of the river at that time and place is called the flood stage.

14.5.2 Flood Profiles. Following a period of increased precipitation or snowmelt, stream discharge rises and then falls over the following days or weeks. There is a delay, or lag time, between the precipitation event and the peak flow of a flood because it takes time for the water to move into stream channels. The length of this delay depends on several factors, including the size of the drainage basin feeding the stream. Under some conditions, the lag time between precipitation and flooding can be quite short, leading to flash floods and heightening the danger to local populations. Flash floods are characteristic of streams draining small watersheds with steep slopes. These streams have short lag times, of only one or two hours, and with intense rainfall quickly rise to a high level.

14.5.3 Urbanization and Streamflow. The growth of cities and suburbs affects the flow of small streams in two ways. First, it becomes far more difficult for water to infiltrate the ground, which is more widely covered by buildings, driveways, walks, pavements, and parking lots. A second change caused by urbanization comes from the introduction of storm sewers, systems of large underground pipes designed to transport storm runoff quickly from paved areas directly to stream channels for discharge. These systems not only shorten the time it takes runoff to travel to channels, they also increase the proportion of runoff by the expansion in impervious surfaces. Together, these changes shorten the lag time of urban streams and heighten their peak discharge levels.

14.6 Lakes: A lake is a body of standing water with an upper surface that is exposed to the atmosphere and does not have an appreciable gradient. Ponds, marshes, and swamps with standing water can all be included under the definition of a lake. Lakes receive water from streams, overland flow, and groundwater, and so they form part of drainage systems. Lakes serve as vital reservoirs of freshwater on the land. They are formed in many different ways but are generally short-lived over geologic time.

14.6.1 The Great Lakes. The Great Lakes—Superior, Huron, Michigan, Erie, and Ontario—along with their smaller bays and connecting lakes, comprise a vast network of inland waters in the heart of North America. They contain 23,000 km 3 (5500 mi 3) of water—about 18 percent of all the fresh surface water on Earth. The Great Lakes are a vast North American water resource, although, they have suffered somewhat from water pollution.

14.6.2 Saline Lakes and Salt Flats. In arid regions, we find lakes with no surface outlet.

In these water bodies, the average rate of evaporation balances the average rate of stream inflow. When the rate of inflow increases, the lake level rises and its surface area broadens, allowing more evaporation and thus striking a new balance. Conversely, if the region becomes more arid, reducing input and increasing evaporation, the water will fall to a lower level. Salt often builds up in these lakes. Streams bring dissolved solids into the lake, and since evaporation removes only pure water, the salts remain behind.

The salinity, or “saltiness,” of the water slowly increases. Eventually, the salinity level reaches a point where salts are precipitated as solids. In some cases the water is missing.

In regions of high evapotranspiration and low precipitation, instead of lakes we find shallow, empty basins covered with salt deposits. These are called salt flats or dry lakes.

14.6.3 Desert Irrigation. Irrigating the desert is a practice as old as civilization itself. Two of the earliest civilizations, Egypt and Mesopotamia, relied heavily on large supplies of water from nondesert sources to irrigate their land. The ancient water sources for Egypt and Mesopotamia were the rivers that cross the desert but derive their flow from regions that have a water surplus. These are referred to as exotic rivers because their flows are derived from an outside region. Salinization and waterlogging are undesirable side effects of long-term irrigation. Arid regions watered by exotic rivers are most affected.

14.7 Freshwater as a Natural Resource: Human society is heavily dependent on fresh surface water for irrigation, drinking water, and industrial usage. However, freshwater is a limited resource.

14.7.1 Water Access and Supply. Despite the abundance of rainfall in many areas, access to freshwater is becoming a chronic problem. Our heavily industrialized societies require enormous supplies of freshwater to sustain them, and the demand continues to rise.

Urban dwellers in developed nations consume 150 to 400 L (50 to 100 gal) of water per person per day in their homes. We use large quantities of water in air conditioning units and power plants, much of it obtained from surface water. lobal water is a finite resource, and in the long term, we can use only as much water as is supplied by precipitation. Desalination, a process that separates freshwater from seawater, offers an

additional source of freshwater in locations lacking sufficient precipitation, but the high energy cost of operating desalination plants precludes their use in most of the developing world.

14.7.2 Pollution of Surface Water. Water pollutants include various types of common ions and salts, as well as heavy metals, organic compounds, and acids. Excessive plant nutrients in runoff feeding lakes can lead to eutrophication—the overgrowth of algae and other aquatic plants in streams and lakes, caused by the accumulation of phosphate and nitrate.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (1‐800‐255‐8629). The Hydrologic Cycle. DVD DV‐3685, 21 minutes.

Underground Water. DVD DV‐3686, 20 minutes. Films for the Humanities and Sciences, Hamilton, NJ (800‐257‐5126).

Water for the Fields. DVD OVW 330‐73‐KS; VHS OVW 330‐73‐A; 27 minutes, 2003.

Water for the Cities. DVD OVW 330‐74‐KS; VHS OVW 330‐74‐A; 27 minutes, 2003.

Insight Media, New York, NY (800‐233‐9910). Human Impact on Surface and Subsurface Waters. DVD #8AS‐2395, 30 minutes, 2008.

Restoring Damaged Rivers. DVD #8AS‐2450, 30 minutes, 2008.

Select Reference Books: Arnell, Nigel. Hydrology and Global Environmental Change. New York, NY: Prentice Hall, 2002.

Bedient, Philip B., Wayne C. Huber, and Baxter E. Vieux. Hydrology and Floodplain Analysis, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2008.

Brooks, Kenneth N., Peter F. Ffolliott, Hans M. Gregersen, and Leonard F. DeBano. Hydrology and the Management of Watersheds, 3rd ed. Ames, IA: Iowa State Press, 2003.

Cech, Thomas V. Principles of Water Resources: History, Development, Management, and Policy, 3rd ed. Hoboken, NJ: Wiley, 2010.

Dzurik, Andrew A. Water Resources Planning, 3rd ed. Lanham, MD: Rowman and Littlefield, 2003.

Gleick, Peter H., Heather Cooley, Michael J. Cohen, Mari Morikawa, Jason Morrison, and Meena Palaniappan. The World’s Water: 2008−2009: The Biennial Report on Freshwater Resources. Washington, DC: Island Press, 2009.

Glennon, Robert. Water Follies: Groundwater Pumping and the Fate of America’s Fresh Waters.

Washington, DC: Island Press, 2002.

Pearce, Fred. Keepers of the Spring: Reclaiming Our Water in an Age of Globalization.

Washington, DC: Island Press, 2004.

Powell, James L. Dead Pool: Lake Powell, Global Warming, and the Future of Water in the West. Berkeley, CA: University of California Press, 2008.

Spellman, Frank R. The Science of Water: Concepts and Applications, 2nd ed. Boca Raton, FL: CRC Press, 2008.

Thorson, Robert M. Beyond Walden: The Hidden History of America’s Kettle Lakes and Ponds.

New York, NY: Walker & Company, 2009.

Whitely, John M., Helen Ingram, and Richard Perry (eds.). Water, Place, and Equity. Cambridge, MA: The MIT Press, 2008.

Younger, Paul L. Groundwater in the Environment. Malden, MA: Blackwell Publishing, 2007.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 15: Landforms Made by Running Water Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Discuss the processes of erosion, transportation, and deposition.

2) Describe stream gradation and how stream valleys evolve.

3) Describe the landforms created by flowing water.

4) Identify the unique landforms of arid climates.

This chapter discusses running water (channelized flow) and the various erosional and depositional processes associated with it. Two of the key factors that control these processes are: 1) the velocity of the water and 2) the size of material being moved by it. A good understanding of the interrelationships between these two factors, size and speed, provides the basis for understanding why material in a stream is either being eroded or deposited, as well as the type of landforms that result.

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Erosion—The removal of material from Earth’s surface by natural processes, such as running water.

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Transportation—The movement of material across Earth’s surface by natural processes, such as running water.

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Deposition—The accumulation (addition) of material on Earth’s surface by natural processes, such as running water.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

USGS Current Water Data for the Nation. This government website displays real-time information, such as discharge, water depth, and precipitation, for rivers across the United States. http://waterdata.usgs.gov/nwis/rt

EPA Stream Classification. This website describes the use of a flowchart system to classify streams. http://water.epa.gov/scitech/datait/tools/warsss/pla_box05.cfm Erosional and Depositional Features of Running Water. This British site describes the various landforms created by running water. http://elearning.stkc.go.th/lms/html/earth_science/locanada3/303/10_en.htm Streams and Drainage Systems. A site maintained by Dr. Stephen Nelson of Tulane University that discusses all major aspects of channelized flow. http://www.tulane.edu/~sanelson/geol111/streams.htm Stream Discharge, Fluvial Sediments and Flood Recurrence Frequencies. A site maintained by Dr. Jonathan Gourley of Trinity College that explains how to calculate stream discharge and flood recurrence intervals. http://www.trincoll.edu/~jgourley/GEOS%20112%20home.htm Channel Processes: Stream Channel Succession. An EPA website that discusses the dynamic evolution of stream channels. http://water.epa.gov/scitech/datait/tools/warsss/successn.cfm Chapter Lecture: 15.1 Erosion, Transport, and Deposition: Most of the world’s land surface has been sculpted by running water, which acts to shape landforms through three closely related processes: erosion, transportation, and deposition. The landforms shaped by the progressive removal of bedrock are called erosional landforms. Fragments of soil, regolith, and bedrock that are removed from the parent rock mass are transported and deposited elsewhere, where they take shape as an entirely different set of surface features—the depositional landforms.

15.1.1 Slope Erosion. Fluvial landforms are shaped by the fluvial processes of overland flow and streamflow. Soil erosion occurs when overland flow transports soil particles downslope. Erosion is greatest on bare slopes of fine particles, carving rills and gullies.

A vegetation cover greatly reduces soil erosion.

15.1.2 Sediment Yield. To compare erosion rates, we use sediment yield, a technical term for the rate of sediment removal in metric tons per hectare per year (tons per acre per year). Both surface runoff and sediment yield are much lower for vegetated surfaces.

In fact, sediment yield from cultivated land under poor management can be more than 10 times greater than that of pasture and about a thousand times greater than that of a pine plantation.

15.1.3 Stream Erosion. Streambeds and banks are eroded by hydraulic action, abrasion, and corrosion. Abrasion by stones on a bedrock riverbed can dig deep depressions known as potholes.

15.1.4 Stream Transportation. Streams carry dissolved matter, sediment in suspension, and a bed load of larger particles that bump and roll along the bottom. A stream’s capacity to carry sediment increases sharply with its velocity.

15.1.5 Deposition. Deposition typically occurs where the velocity of streamflow decreases. For example, deposition occurs along stream banks when the streamflow slows down on the inside of a bend in the channel. During flooding, fast-moving floodwaters slow down and spread out over the valley floor, depositing alluvium in layers.

Fine sediment, rich in organic matter, can improve soil fertility; however, flooding can sometimes also leave behind sterile layers of sand or gravel.

15.2 Stream Gradation and Evolution. Most major stream systems have experienced thousands of years of runoff, erosion, and deposition. Now that you have seen how these processes occur, let us take a closer look at the part they play in the evolution of streams over time.

15.2.1 Stream Gradation. Over time, a stream develops a graded profile in which the gradient is just sufficient to carry the average annual load of water and sediment produced by its drainage basin.

15.2.2 Evolution of Stream Valleys. Through the process of gradation, streams and their valleys evolve in a predictable way. In the early stages of stream evolution, streams experience sudden changes in gradient, where faulting and tectonic uplift lead to a sudden drop-off, or where the resistance of streambed materials suddenly changes. Points at which the gradient of the stream changes abruptly, such as waterfalls and rapids, are called nickpoints. Streamflow is particularly fast and turbulent at nickpoints, so these segments of the stream are more rapidly eroded back to the average gradient of the stream, in a process called downcutting. A stream may erode its streambed rapidly through downcutting in the early stages of its evolution, but eventually the balance between erosion and deposition stabilizes. There is a lower limit to how far a stream can erode its bed, called its base level. The hypothetical base level for all streams is sea level.

Suggestion: It is useful to remind the class that it takes tens of millions of years for the initial ungraded landscape to undergo all of the erosional and depositional stream processes that result in a graded condition.

15.2.3 Stream Rejuvenation. Tectonic uplift sometimes raises the gradient of portions of a graded stream. Faulting, for example, may push up blocks of crust to form waterfalls and rapids. With new, steeper segments, the stream begins a new cycle of downcutting.

Such streams are said to be rejuvenated streams. Where rapid uplift causes meandering rivers to cut deeply into bedrock, entrenched meanders are formed.

15.2.4 Theories of Landscape Evolution. The geomorphic cycle traces the fate of rivers and fluvial landforms from an initial uplift that creates steep slopes and canyons to a final low, gently rolling surface called a peneplain. The equilibrium approach sees fluvial landforms as reflecting a balance between the processes of uplift and denudation acting on rocks of varying resistance to erosion.

15.3 Fluvial Landforms: Fluvial landforms are Earth-surface features produced by running water.

15.3.1 Meandering Streams and Floodplains. Meandering is a characteristic of graded rivers carrying substantial loads of sediment of varying sizes. In a meander bend, water moves with greater velocity on the outside of the bend, where the channel is deeper. The greater velocity erodes the floodplain sediment, creating a cut bank and causing the bend to grow outward or in a downstream direction On the inside of the bend, the flow is slower, and sediment accumulates in a point bar. Sometimes the streamflow cuts off a meander loop by eroding through the narrow portion of the meander neck. After the cutoff, silt and sand are deposited across the ends of the former channel, producing an oxbow lake. Eventually, the lake fills in with sediment and becomes first a swamp and then a meander scar. Cutoffs are sometimes man-made to straighten the stream channel and make it more convenient for transport. Over time, the erosion and deposition occurring as river meanders grow and move downstream creates a broad, flat floodplain of alluvium.

The floodplain is normally flooded each year or two, when streamflow increases and the river leaves its banks. As the velocity of the water flowing away from the bank and across the floodplain decreases, the sediment begins to settle out. Sand and silt accumulate first, building up natural levees along the channel. Farther away, fine sediment settles out of the nearly stagnant water, accumulating between the levees and the bluffs that bound the floodplain, an area known as the backswamp.

15.3.2 Braided Streams. Braided streams develop when an aggrading stream carries a large volume of bed load and has seasonal fluctuations in discharge.

15.3.3 Deltas. Deltas are found where rivers carry sediment into lake or ocean basins. As the streamflow enters the water body, its velocity slows, and it drops its load.

Suggestion: Students are likely to be familiar with area rivers and streams. Have them try to recognize these erosional and depositional features the next time they journey to or across a local river or stream and discuss them in class. Also mention that they can easily spot numerous meandering streams the next time they are in an airplane.

15.4 Fluvial Processes in an Arid Climate: Although rain falls infrequently in desert environments, running water shapes desert landforms with great effectiveness because of the lack of vegetation cover.

15.4.1 Alluvial Fans. Alluvial fans are common features of arid landscapes. They occur where streams discharge water and sediment from a narrow canyon or gorge onto an adjacent plain.

15.4.2 Mountainous Deserts. Where tectonic activity has recently produced block faulting in an area of continental desert, the assemblage of fluvial landforms is particularly diverse and interesting. The Basin-and-Range Province of the western United States, which encompasses large parts of Nevada and Utah, southeastern California, southern Arizona, and New Mexico, is an example. Landforms of mountainous deserts include alluvial fans, dry lakes or playas, and pediments—rock platforms veneered with alluvium.

Suggestion: As students travel during and after college, they may come across dry streambeds under bridges in arid regions of the American Southwest. If a thunderstorm were to occur during their visit, the rapidity with which the water rises in the stream channel is quite spectacular to see, and often dangerous.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (1‐800‐255‐8629). Running Water 1: Rivers, Erosion, and Deposition. Running Water 2: Landform Evolution. VHS VC‐3527, 30 minutes each.

Films for the Humanities and Sciences, Hamilton, NJ (1‐800‐257‐5126). Processes That Shape the Earth. DVD PCT 397‐73‐K; VHS PCT 397‐73‐A; 32 minutes, 2010.

Insight Media, New York, NY (800‐233‐9910). Geomorphology: Study of the Shape. DVD #79‐AM‐732, 20 minutes, 2005.

Rivers Shapers of Earth Landscapes. DVD #79‐AM‐470, 23 minutes, 2001.

Select Reference Books: Rose, Calvin. An Introduction to the Environmental Physics of Soil, Water and Watersheds. New York, NY: Cambridge University Press, 2004.

Strand, Ginger. Inventing Niagara: Beauty, Power, and Lies. New York, NY: Simon & Schuster.

2008.

Ward, Andy D. and Stanley W. Trimble. Environmental Hydrology, 2nd ed.. Boca Raton, FL: Lewis Publishers, 2004.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 16: Landforms Made by Waves and Wind Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Describe the effects of waves and tides.

2) Describe the erosional and depositional landforms of coasts.

3) Describe how wind shapes landforms.4) Describe types of sand dunes and loess.

Coastlines and deserts, although very distinctive environments, contain many unique and interesting features for students to study and appreciate. The following web sites not only provide examples of such features but also discuss the problems associated with these environments, such as desertification and coastal erosion.

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Tides: Tides can often be a confusing topic to students, especially when comparing the influence of the Sun and the Moon. An animation in WileyPLUS may prove useful either in class or as an out-of-class study aid. In addition, the NOAA site, Our Restless Tides, provides further information about tides. Have the class use the internet to examine daily tidal changes at an East Coast and a West Coast site. There are numerous websites that monitor tidal changes.

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Visualizing Coastal and Desert Features: Numerous internet sites provide photographic and illustrative examples of desert and coastal features. Use sites such as the Lisa Wells’ site with her extensive array of photographs to provide a broad range of examples. Where relevant, challenge students to find local examples.

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Deserts: Students who have never visited a desert tend to have strong misconceptions about this type of environment, often assuming that they all have extensive areas of deep sand. Assign students to find out about the broad array of deserts, using, for example, the USGS site. Challenge them to compare and contrast deserts from various regions.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

Coastal America. A detailed site about many aspects associated with coastlines including erosion and vegetation. http://www.coastalamerica.gov/ Our Restless Tides. A general introduction by NOAA to the principles of tides and how they work. http://co‐ops.nos.noaa.gov/restles1.html Coastal Erosion Fieldtrip—San Diego County, California. A well-illustrated virtual field trip with photographs and explanations of coastal erosion. http://www.miracosta.cc.ca.us/home/cmetzler/field_trip/top.html Carolina Coastal Science. A site focusing specifically on the coasts of the Carolinas with particular reference to the problems they face, such as hurricanes. http://www.ncsu.edu/coast/ Deserts from USGS. A brief introduction to the features and processes associated with deserts. http://pubs.usgs.gov/gip/deserts/ Desert USA. A wide variety of resources concerning deserts, including links to many specific desert sites. http://www.desertusa.com/index.html Images illustrating the principles of geomorphology by Lisa Wells. This site contains many photographs of coastal and desert features, as well as of glaciers and river landforms. http://geoimages.berkeley.edu/GeoImages/Wells/wells.html

Chapter Lecture: 16.1 The Work of Waves and Tides: Wind and waves are the key agents in making landforms at the interface between land and sea. This extraordinarily dynamic boundary is referred to as the shoreline, whereas the coastline refers to the area where coastal processes operate.

16.1.1 Waves. Waves are driven by wind. Their height is related to the speed of the wind, its duration in time, and the fetch, which is the distance that the wind has blown over the water. Very large waves can develop if there is a combination of strong winds and long fetches. Wave height can rise very quickly if there is a substantial and sustained increase in wind speed. The energy contained in the wave is expended as breakers that push a swash, loaded with sand and gravel, onto the beach. Water and sediment are returned to the sea as backwash.

16.1.2 Littoral Drift. Wave action can build beaches that are usually made up of sand, but some beaches are made of pebbles, cobbles, and even shell fragments. Breakers that approach the shore at an angle can produce littoral drift. Littoral drift can also include beach drift, which is sediment movement along the beach, and longshore drift, which is sediment moving just offshore of the beach. In order to prevent retrogradation, which is a reduction in width of the beach, groins, which are walls or embankments constructed at

right angles to the shoreline, can be built to help trap littoral drift and thereby reduce the loss of beach area.

16.1.3 Tides. Ocean tides are created by the gravitational attraction of the Moon on ocean waters, as well as the rotation of the coupled Earth-Moon system around its common center of mass. When the Earth, Sun, and Moon are aligned, their gravitational forces combine to produce a higher tide called a spring tide. In contrast, when the Moon and Sun are positioned at right angles to each other, this results in a lower tide, called a neap tide. Ocean tides produce tidal currents at the shoreline. These currents scour inlets and distribute fine sediment in bays and estuaries.

Suggestion: Discuss with the class the extreme tidal variations that occur in the Bay of Fundy, Gulf of Maine, bounded by the State of Maine and the Canadian provinces of New Brunswick and Nova Scotia. Why is this area prone to the highest tides in the world?

16.1.4 Tsunamis. A tsunami is a gigantic ocean wave caused by an earthquake or volcanic explosion. The Indian Ocean tsunami of 2004 killed more than 200,000 people and laid waste to coastlines from Indonesia to Tanzania. A tsunami originates when a sudden movement of the seafloor generates a succession of water waves. These waves travel over the ocean at 700 to 800 km/hr (435 to 500 mi/hr), moving outward in all directions from their source. Ocean waters rush landward and surge far inland, destroying coastal structures and killing inhabitants as they pass, at speeds of up to 15 m/s (34 mi/hr) for several minutes.

16.2 Coastal Landforms: As waves break upon the shore, they expend tremendous amounts of energy. This energy, along with the coastal currents it produces, shapes coastlines by eroding steep cliffs, carving out bays, and building beaches. Two terms are important to distinguish in describing coastal landforms and processes. The shoreline refers to the dynamic zone of contact between water and land. The coastline (or coast) refers to the zone of shallow water and nearby land that fringes the shoreline. It is the zone in which coastal processes shape landforms.

16.2.1 Erosional Coastal Landforms. Waves erode weak materials, resulting in marine scarps, and weather-resistant rocks, which are reshaped as marine cliffs. Caves, arches, stacks, and abrasion platforms are landforms of marine cliffs. Where resistant rocks meet the waves, sea cliffs often occur. At the base of a sea cliff is a notch, carved largely by physical weathering. As the cliff erodes, the shoreline gradually retreats shoreward. Sea cliff erosion results in a variety of erosional landforms, including sea caves, sea arches, and sea stacks. The retreat of the cliffs leaves behind a broad, gently sloping plain, called a shore platform, at the base of the cliffs. If tectonic activity elevates the coastline or if sea level falls, the platform is abruptly lifted above the level of wave attack. What was a shore platform is raised up and becomes a marine terrace.

16.2.2 Depositional Coastal Landforms. Most of the sediment we find along a coastline is provided by rivers that reach the ocean. Waves then transport and deposit this sediment to take shape as shoreline features such as beaches, bars, and spits. These

depositional landforms are relatively transitory, however, appearing, disappearing, or migrating as a result of seasonal changes, storms, and human engineering. A beach is a wedge-shaped sedimentary deposit, usually of sand, built and worked by wave action.

The face of a beach varies over time as waves either deposit or erode more sand. Coastal foredunes form a protective barrier against storm wave action, preventing waves from overwashing a beach ridge or barrier island. Where littoral drift moves the sand along the beach toward a bay, the sand is carried out into the open water, extending like a long finger, called a spit. As the spit grows, it forms a barrier, called a baymouth bar, across the mouth of the bay. Once the bay is isolated from the ocean, it is transformed into a lagoon. And where a spit grows to connect the mainland to a near-shore island, it forms a tombolo. Last, barrier islands of the coastal plain were formed as sea level rose after the last glaciation. Behind the island is a shallow lagoon that is serviced by tidal inlets flowing across the barrier island.

16.2.3 Coastlines of Submergence. Coastlines of submergence are formed when rising sea level partially drowns a coast or when part of the coast sinks. At the end of the last glaciation, sea level rose by about 120 m (394 ft), submerging coastal landscapes. Ria and fiord coasts result from submergence of a landmass. Islands, bars, estuaries, and bays are characteristic of these “drowned” coastlines. Climate scientists expect sea level to rise even higher over the next century as a result of global climate change. Today, many of the major river delta areas are experiencing submergence resulting from land use practices and sea-level rise.

16.2.4 Coral Reefs. Coral reef coasts develop in warm oceans, where corals build reefs at the land-sea margin. Fringing reefs, barrier reefs, and atolls are three types of coral reef coasts. Fringing reefs build up as platforms attached to shore. They are widest in front of headlands where the wave attack is strongest. Barrier reefs lie out from shore and are separated from the mainland by a lagoon. At intervals along barrier reefs, there are narrow gaps through which excess water from breaking waves is returned from the lagoon to the open sea. Atolls are more or less circular coral reefs that enclose a lagoon with no landmass inside. Most atolls are rings of coral growing on top of old, sunken, hotspot volcanoes.

16.2.5 Coastal Engineering. Around the world, human populations have always settled along coasts, drawn by ocean-related industries such as shipping, fishing, and tourism, as well as by the scenic views and coastal climates. More than 70 percent of the world’s population lives on coastal plains, and 11 of the world ’s 15 largest cities are on coasts or estuaries. Human efforts to control dynamic coastal processes in order to protect their property and investments are often futile. Worse, they can disrupt the balance of sediment supply and erosion. Homeowners sometimes construct seawalls or pile up mounds of boulders, called rip-rap, in an attempt to absorb some of the destructive, erosive energy of the waves and protect their property. Property owners and/or governments sometimes try to build a broad, protective beach through a process called artificial beach nourishment. One method of beach nourishment is simply to pump sand from offshore onto the beach. Another way to protect beaches is to try to prevent the loss of sand by trapping it as it is transported down the shore by littoral drift. This is accomplished by

installing walls or embankments, called groins. Groins are walls or embankments built at right angles to the shoreline. They trap littoral drift and help prevent beach retrogradation.

16.3 Wind Action: The term eolian denotes some aspect of wind activity. This activity usually involves the movement of mineral particles when they are dry and unprotected by vegetative cover. Suitable conditions for the development of these eolian landforms are found in the deserts and semiarid areas of the world as well as on sandy coastlines.

16.3.1 Erosion by Wind. Erosion by wind involves two major processes: 1) Abrasion— the process by which wind moves sand and dust particles against an exposed rock or soil Surface (sandblasting) and 2) Deflation—the process that removes loose soil or sediment particles from the ground surface, producing a shallow depression called a blowout. Wind deflation can also result in the creation of a desert pavement, i.e., a surface armor of coarse particles that may reduce continued deflation if vehicles are not driven on the surface.

16.3.2 Transportation by Wind. Wind transports sand near the surface by saltation and surface creep. Silt and clay are carried aloft by atmospheric turbulence in dust storms.

Saltation is the process by which sand grains are moved. They are carried by the wind in low arcs, perhaps a meter long, from one point to another. Each time they hit the surface, they force other particles into the air, which are then carried downwind. At higher wind speeds, sand grains are also dragged along the ground, in the same way that a stream carries bed load. This movement is called surface creep.

16.4 Eolian Landforms: Eolian landforms occur where surface mineral particles are dry and unprotected by a vegetation cover. These conditions are found in deserts and semiarid regions of the world, as well as on sandy shorelines.

16.4.1 Sand Dunes. Sand dunes move as wind-blown sand grains hop up the gentle windward side of the dune and then fall onto the steeper slip face of the leeward side.

16.4.2 Types of Dunes. Sand dunes occur when a source, such as a beach or an outcrop of sandstone, can furnish enough sand so that it can be moved by the winds. Dune type depends on the availability of the sand supply, strength of the wind, and the amount of vegetation present. There are five major dune forms. Barchan dunes have the shape of a crescent with downward directed points. They can be arranged individually or in chains that lead away from the source of sand. Transverse dunes have a wavelike form and need a very large supply of sand. Their crests are often at right angles to the direction of the dominant wind. Star dunes are large hills of sand that tend to remain fixed in place. These dunes have radiating arms that resemble a star and are common features in the Sahara and Arabian deserts. Parabolic dunes are found on semiarid plains. They are arc‐shaped with points directed in the direction of the prevailing wind. On occasion, they can be drawn out into hairpin dunes that migrate downwind. Longitudinal dunes are long and low sand ridges that form parallel to the prevailing wind direction. They cover large areas of tropical and sub‐tropical deserts in Africa, Australia, and portions of the Colorado Plateau.

Coastal foredunes are irregularly shaped hills and depressions that are formed landward of the beach. They are partially covered with vegetation and thereby form a protective barrier against storm wave action that prevents waves from washing over a beach ridge or barrier island.

16.4.3 Loess. Loess is a deposit of wind‐blown silt that can reach thicknesses of over 100 ft (30 m). The thickest deposits are in northern China and presumably were derived from areas in the interior of Asia. The loess deposits in the U.S. and Europe represent silt that came from fresh glacial deposits during the Pleistocene epoch. It is easily eroded by water and wind. Loess soils (mollisols) are desirable for farming operations.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (1‐800‐255‐8629). Waves, Coastlines, and Beaches. DVD DV‐3684, 18 minutes, 2000.

Films for the Humanities and Sciences, Hamilton, NJ (1‐800‐257‐5126). Oceans and Seas. DVD PCT‐347‐24‐K, VHS PCT‐347‐24, approximately 22 minutes, 2006.

Insight Media, New York, NY (800‐233‐9910). On the Coast. DVD #8A‐S‐2151, 27 minutes, 2003.

Insight Media, New York, NY (800‐233‐9910). Waves, Tides, and the Coastal Environment.

DVD #79‐AM‐452, 24 minutes, 2001.

Select Reference Books: Bird, Eric. Coastal Geomorphology: An Introduction, 2nd ed. Hoboken, NJ: John Wiley and Sons, Inc, 2008.

Cooke, Ronald U., Andrew Warren, and Andrew S. Goudie. Desert Geomorphology, 2nd ed.

London, UK: Taylor and Francis, 1993.

Smithson, Peter, Ken Addison, and Ken Atkinson. Fundamentals of the Physical Environment, 3rd ed. New York, NY: Routledge, 2002.

Trujillo, Alan P. and Harold V. Thurman. Essentials of Oceanography, 10th ed. Upper Saddle River, NJ: Prentice Hall, 2011.

Instructor’s Manual for Strahler, Introducing Physical Geography, 6th Edition Chapter 17: Glacial Landforms and the Ice Age Chapter Objectives: Upon completion of this chapter, the student will be able to: 1) Identify the types of glaciers.

2) Discuss the processes of glacial action.

3) Describe the landforms made by alpine glaciers and ice sheets.

4) Describe periglacial processes and their landforms.

5) Describe the causes and environment of an ice age.

The broad array of processes and features associated with glaciers often prove challenging to students. The following websites provide many images that are useful to explain features and processes, while sites such as Cracking the Ice Age cover the nature of glaciations. Visualizing Glacier Features offers a wide range of examples and illustrations to help students understand the various concepts. Use sites such as the All About Glaciers site from NSDIC or the Illustrated Glossary of Alpine Glacial Landforms to provide a broad range of examples. Where relevant, challenge students to find local examples. In order to understand the last ice age, review the time scale and then assign your students to place the last ice age in context. The vast size of the Antarctic can make it a difficult environment to envisage. A few websites, such as the Glacier site from Rice University, provide many images and facts concerning this important ice sheet.

The current concern for the future of the ice sheet in a warming world is an interesting topic for debate. Challenge students to investigate and bring their opinions to class.

Relevant Internet Resources: ConceptCaching.com is a website that promotes student spatial awareness by relating specific features on Earth’s surface with their visual character and GPS coordinates. Photographs and GPS coordinates that demonstrate core concepts in geography are “cached” for viewing by core concept and by region.

GeoDiscoveries animations and interactivities reinforce and illustrate key concepts. They are good for use in lectures and demonstrations. Students can use the activities for practice or study.

All About Glaciers (NSIDC). An extensive introduction to the features and processes associated with glaciers. http://nsidc.org/glaciers/ Illustrated Glossary of Alpine Glacial Landforms. An extensive array of images of alpine glacial landforms. http://www.uwsp.edu/geo/faculty/lemke/alpine_glacial_glossary/

Nova—Cracking the Ice Age. A web site developed for the Nova TV program that presents evidence concerning the last ice age and what that evidence can tell us about how climate works. http://www.pbs.org/wgbh/nova/ice/ The Cryosphere from the National Snow and Ice Data Center. A general introduction to the processes and features associated with the cryosphere. http://nsidc.org/cryosphere/index.html Chapter Lecture: 17.1 Types of Glaciers: Almost 70% of Earth’s freshwater is stored in the cryosphere, or realm of ice and snow, primarily in the form of glaciers. Glaciers form in regions that have low temperatures and sufficient snowfall, conditions found at both high elevations and high latitudes.

In mountains, glacial ice can develop even in tropical and equatorial zones if the elevation is high enough to keep average annual temperatures below freezing. Glaciers can take a variety of forms, but they can generally be divided into two broad categories: alpine glaciers and ice sheets, which are also called continental glaciers.

17.1.1 Alpine Glaciers. Alpine glaciers, or mountain glaciers, are found in high mountain ranges where snow accumulates and temperatures are cold enough to maintain yearround snow cover. Alpine glaciers originate from a snowfield that accumulates in a bowl-shaped depression called a cirque. When alpine glaciers are contained within these basins, they are called cirque glaciers. However, most alpine glaciers flow out of these basins to become valley glaciers, which occupy sloping stream valleys between steep rock walls. When a valley glacier flows out onto a surrounding plain, it appears as a piedmont glacier. When a valley glacier terminates in seawater, as a tidewater glacier, blocks of ice break off to become icebergs.

17.1.2 Ice Sheets. In arctic and polar regions, temperatures are low enough year-round for snow to collect over broad areas, eventually forming a vast layer of glacial ice. Snow begins to accumulate on uplands, which are eventually buried under enormous volumes of ice. The layers of ice can reach a thickness of several thousand meters. The ice is thickest at the interior and thins toward the margins. The ice then spreads outward, over surrounding lowlands, and covers all landforms it encounters. We call this extensive type of ice mass an ice sheet, or a continental glacier. At some locations, ice sheets extend long tongues to reach the sea, known as outlet glaciers.

17.1.3 Ice Shelves, Sea Ice, and Icebergs. In Antarctica and Greenland, ice sheets meet the sea, where they become large plates of floating glacial ice called ice shelves. Ice shelves are fed by the ice sheets, and they also accumulate new ice through the compaction of snow. Free-floating ice on the sea surface takes two forms: sea ice and icebergs. Sea ice is molded by direct freezing of ocean water and accumulation of snow atop the ice. The surface zone of sea ice is composed of freshwater, while the deeper ice is salty. Icebergs are masses of ice that have broken free from alpine glaciers, terminating in the ocean, or from floating ice shelves. This process is called calving of the glacier or ice shelf. Icebergs float very low in the water because they are only slightly less dense than seawater.

17.2 Glacial Processes: Although many parts of the world have snow and ice, only certain regions can produce glaciers. And whether glaciers survive and advance is dependent on specific climatic conditions. Scientists who study the formation and behavior of glaciers are called glaciologists.

17.2.1 Formation of Glaciers. Glaciers develop as snow accumulates from year to year.

When the snowfall of the winter exceeds the loss of snow in the summer due to evaporation and melting, a new layer of snow is added to the surface of the glacier.

Surface snow melts and refreezes on warm days and cold nights, compacting the snow and turning it into granular ice called firn. This intermediate ice is then compressed over a period of 25 to 100 years into hard, crystalline glacial ice by the weight of the layers above it. The mass balance of a glacier depends on snow input in the zone of accumulation, and melting and evaporation in the zone of ablation.

17.2.2 Movement of Glaciers. When snow accumulates to a great thickness, it can turn into flowing glacial ice. Flow is always downward, forward, or outward in response to gravity.

17.2.3 Glacial Erosion and Deposition. Glacial ice normally contains rock and sediment that it has picked up as it flows. These rock fragments range from large angular boulders to pulverized rock flour. Most of this material is composed of loose rock debris and sediments found on the landscape as the ice overrides it. Alpine glaciers also carry rock debris that slides or falls from valley walls onto the surface of the ice. But glaciers can create their own sediment, too, by eroding underlying bedrock. Glaciers don’t generate enough pressure to fracture bedrock directly, but they can loosen and pluck out bedrock blocks that are already fractured and easy to split off. The glacier finally deposits the rock debris at its terminus, where the ice melts. We use the term glacial drift to refer to all the varieties of rock debris that are deposited by glaciers. There are two types of drift.

Stratified drift consists of layers of sorted and stratified clays, silts, sands, or gravels.

These materials were deposited by meltwater streams or in bodies of water adjacent to the ice. Till is an unstratified mixture of rock fragments, ranging in size from clay to boulders, that is deposited directly from the ice, without water transport. Glaciers drop debris along their margins and where melting occurs in the form of a moraine.

17.3 Glacial Landforms: Glaciers create both erosional and depositional landforms in mountain ranges and in areas formerly covered by continental ice sheets. During the 2.5 million years of the present Ice Age, Earth has experienced dozens of glaciations. Many parts of northern North America and Eurasia have been covered many times by massive sheets of glacial ice. As a result, glacial ice has shaped many landforms now visible in regions from the midlatitudes to subarctic zones.

17.3.1 Landforms Made by Alpine Glaciers. Alpine glaciers typically erode existing valleys down to hard bedrock, stripping them of regolith developed by weathering and mass wasting. Valley heads are enlarged and hollowed out by glaciers, producing

bowl-shaped cirques. A cirque marks the origin of the glacier and is the first landform produced. When a glacier carves a depression into the bottom of a cirque and then melts away, a small lake called a tarn can take shape. Over time, glacial erosion of bedrock and mass wasting of adjacent slopes steepen the sides of the cirque. Intersecting cirques carve away the mountain, leaving peaks called horns and sharp ridges called arêtes.

Where opposing cirques have intersected and eroded deeply, a notch called a col forms.

Alpine glaciers strip valleys of their soil, regolith, and sediment to form glacial troughs.

A glacial trough leading into the ocean is a fiord.

17.3.2 Landforms Made by Ice Sheets. Like alpine glaciers, ice sheets strip away surface materials and erode bedrock. Ice sheets are also responsible for depositional landforms.

Like huge conveyor belts, traveling ice sheets transport and deposit debris. When the glacial margin is stationary over long periods of time, as was the case during the last glaciation, deposited debris accumulates in distinctive patterns. As the glaciers retreat, they leave behind several types of depositional landforms. Sediment deposited in a ridge along the front or side edge of a glacier is known as a moraine. Glacial till and water-laid sediment that accumulates at the front edge of the ice results in an irregular jumbled heap of debris, sand, and gravel called the terminal moraine. When the ice front retreats, it may pause for some time along a number of positions, forming recessional moraines. As glacial ice moves forward, it compacts and compresses the sediment underneath it, creating lodgment till, a layer of particles of all sizes that is usually tough and dense. On top is a layer of melt-out till that is let down as the ice retreats or stagnates and melts in place. It is a lighter and looser deposit that is easier to plow and work. Together, these two layers make-up the till plain behind the terminal moraine. Two features of the till plain are eskers and drumlins. An esker is a sinuous ridge of sand and gravel, developed at the bottom of an ice tunnel or channel that drains the ice sheet. A drumlin is a smoothly rounded, oval hill of glacial till that resembles the bowl of an inverted teaspoon. Beyond the moraine, an outwash plain is formed from water-laid sediment left by streams carrying water away from the ice front. The plain is built up from layers of sand and gravel. Although the plain is generally smooth, it is often marked by kettles and kames. A kettle is a depression, originally formed as outwash sand and gravel builds up around a block of stagnant ice. When the ice block melts, a depression—the kettle hole— remains in the outwash plain. A kame is a hill or mound of water-laid sediment found on the outwash plain.

17.3.3 Human Use of Glacial Landforms. Landforms associated with glaciers are of major environmental importance. Glaciation can have both positive and negative effects on the land for agricultural development. In hilly or mountainous regions such as New England, the glacial till is thinly distributed and extremely stony, making cultivation difficult. Till deposits built up on steep mountains or hillslopes can absorb water from melting snows and spring rains and then become earthflows. Crop cultivation is also hindered along moraine belts because of the steep slopes, the irregularity of the topography, and the number of boulders. Moraine belts are, however, well suited to pastures. Flat till plains, outwash plains, and lake plains, on the other hand, can sometimes provide very productive agricultural land. Stratified drift provides sand and gravel deposits from outwash plains, kames, and eskers that are used for road

construction and in concrete. Thick, stratified drift deposits can also serve as aquifers, which are a major source of groundwater.

17.4 Periglacial Processes and Landforms: In the arctic and alpine tundra regions, year-round cold temperatures keep the ground frozen and prevent the growth of substantial vegetation.

These periglacial regions were formerly covered with ice or near a glacial front. Their presentday climate, which is exemplified by treeless, frozen ground, gives rise to a unique set of periglacial processes and landforms.

17.4.1 Permafrost. In the tundra, mean annual temperatures are below freezing, and only the top layer of soil or regolith thaws during the warmest month or two. Ground and bedrock that lie below the freezing point of freshwater (0°C, 32°F) all year round are called permafrost. Permafrost includes clay, silt, sand, pebbles, and boulders, as well as solid bedrock perennially below freezing. Permafrost is more or less extensive based on climate conditions in a particular region. In the coldest regions of the northern hemisphere, permafrost is continuous; all ground surfaces are frozen, except those beneath deep lakes. The zone of continuous permafrost coincides largely with the tundra climate, but it also includes a large part of the boreal forest climate of Siberia. Permafrost reaches to a depth of 300 to 450 m (about 1000 to 1500 ft) in the continuous zone near latitude 70° N. Discontinuous permafrost is defined as areas where permafrost occurs only under favorable conditions, such as north-facing slopes. These areas are common in much of the boreal forest climate zone of North America and Eurasia. Permafrost terrains have a shallow surface layer, known as the active layer, which thaws with the changing seasons. The active layer ranges in thickness from about 15 cm (6 in.) to about 4 m (13 ft), depending on the latitude and nature of the ground. The thickness of the active layer varies with climate, getting thinner during colder periods and thicker during warmer periods.

17.4.2 Ground Ice and Periglacial Landforms. Water is commonly found in pore spaces in the ground, where, in its frozen state, it is known as ground ice. The amount of ground ice present within and above the permafrost layer varies greatly. At great depth, pores in the rock hold little if any frozen water. Near the surface, ground ice can take the form of a body of almost 100% ice. One type of ground ice is the ice wedge. Ice wedges develop in vertical cracks in permafrost opened by intense winter cold. They are often interconnected as ice-wedge polygons. During the short summer season, when ice in the active layer thaws, the soil and regolith becomes saturated with water that cannot escape downward into solid permafrost. In a process called gelifluction, the saturated soil can slump or flow down shallow slopes and take shape as terraces or lobes. In areas that contain coarse-textured regolith, consisting of rock particles in a range of sizes, this shifting and thrusting can give rise to some very beautiful and distinctive features, including rings and stripes of coarse fragments called patterned ground. Another remarkable ice-formed feature of the arctic tundra is a conspicuous conical mound called a pingo. Pingos emerge when pockets of unfrozen ground under a lakebed freeze after a lake is drained. As the unfrozen groundwater is converted to ice, its volume expands and

pushes upward into a dome shape. In extreme cases, pingos reach heights of 50 m (164 ft), with base diameters of 600 m (about 2000 ft).

17.4.3 Human Interactions with Periglacial Environments. With climate warming, the upper layers of permafrost, which are often rich in ice, begin to melt and release water.

The ground collapses, producing water-filled depressions. And because water conducts heat into the permafrost more effectively than vegetation-covered soil, permafrost thawing expands, and the depressions grow into shallow lakes. Now the landscape resembles limestone karst formations, so we call the new terrain thermokarst. Instead of being shaped by chemical solution, however, thermokarst terrain is generated by heat flow. Clearing of natural surface layers can induce rapid thawing of ice masses in permafrost, leading to thermal erosion and the growth of shallow, thermokarst lakes.

17.5 Global Climate and Glaciation. Glacial ice sheets have a major impact on our global climate.

For starters, glacial ice sheets reflect much of the solar radiation they receive, and so have a direct influence Earth’s radiation and heat balance. The temperature difference between ice sheets and warm regions near the Equator helps drive the global heat transport system. When the volume of glacial ice increases, as it does during a glaciation, sea level falls. When the ice sheets melt away, sea level rises. Today’s coastal environments evolved as sea level rose in response to the melting of the last ice sheets.

17.5.1 History of Glaciation. Glaciation occurs when temperatures fall in regions of ample snowfall, allowing ice to accumulate and build. Deglaciation happens when the ice melts at the beginning of a period of milder climate, called an interglaciation. An ice age is a period of millions of years of generally cold climate consisting of many alternating glaciations and interglaciations. Throughout its long history, Earth has cycled through a number of ice ages. An ice age includes cycles of glaciation, deglaciation, and interglaciation. Earth is presently in an interglaciation. Throughout the past 2.5 million years or so, Earth has been experiencing the Late-Cenozoic Ice Age (or, simply, the Ice Age). This period consists of alternating glacial interglacial periods; five major glaciations have occurred in the past 500,000 years. The last major glaciation, called the Wisconsin glaciation, started about 120,000 years ago. During this glaciation, ice sheets covered much of North America and Europe, as well as parts of northern Asia and southern South America. All high mountain areas of the world developed alpine glaciers.

In North America, one of the most significant aftereffects of the Wisconsin glaciations was the formation of the Great Lakes.

17.5.2 The Holocene Epoch. Since the Wisconsin glaciation ended, the Earth has been in a period of interglaciation called the Holocene epoch. There were three major climatic periods during the Holocene epoch leading up to the last 2000 years. These periods are inferred from studies of fossil pollen and spores preserved in glacial bogs that show changes in vegetation cover over time. The earliest of the three is the Boreal stage, characterized by boreal forest vegetation in midlatitude regions. A general warming followed until the Atlantic stage, with temperatures somewhat warmer than today, was reached about 8000 years ago (–8000 years). Next came a period of below-average temperatures—the Subboreal stage. This stage spanned the age range of –5000 to –2000

years. We can describe the climate of the past 2000 years on a finer scale, thanks to historical records and detailed evidence. A secondary warm period occurred in the period ce 1000 to 1200 (–1000 to –800 years). This warm episode was followed by the Little Ice Age, ce 1450–1850 (–550 to –150 years), when valley glaciers made new advances and extended to lower elevations.

17.5.3 Triggering the Ice Age. The current Ice Age was most likely triggered by the motions of continents, which provided a snow-covered Antarctic continent and an icecovered Arctic Ocean, restricted ocean flow paths, and changed atmospheric circulation.

17.5.4 Cycles of Glaciation. According to the astronomical hypothesis, the timing of glaciations and interglaciations is determined by variations in insolation produced by minor cycles in Earth’s orbit and axial rotation. The cycles are plotted on a graph known as the Milankovitch curve, named for the Serbian engineer and mathematician Milutin Milankovitch, who first calculated the cycles in 1938. The dominant cycle for summer daily insolation at 65° N latitude indicates periods of 40,000 years. However, the combination of Milankovitch variations demonstrates major climate-shifting cycles of 100,000 and 400,000 years. Dating of the deep-ocean cores and Antarctic ice cores confirms the prevalence of these major cycles for the onset of climate change that activates glaciation and deglaciation.

17.5.5 Glaciation and Global Warming. Global temperatures have been slowly warming within the past century, as measured by an international community of meteorologists.

The Antarctic and Greenland Ice Caps are now shrinking. Some Antarctic ice shelves are thinning and fracturing. Arctic sea ice is also thinning, and becoming reduced in extent.

As northern ice and snow have shrunken in extent, their bright surfaces have been replaced by darker ocean water and land surfaces. The albedo of arctic regions has decreased significantly, resulting in the absorption of more solar insolation. This in turn propels a positive feedback loop that further increases warming, which then accelerates deglaciation at both poles, as well as in alpine environments.

Select Audiovisual Aids: Crystal Productions, Glenview, IL (1-800-255-8629). Glaciation: Ice Shapes the Land. DVD DV-3681, 51 minutes.

Crystal Productions, Glenview, IL (1-800-255-8629). Glaciers and Glaciation. DVD DV-3203, 25 minutes.

Films for the Humanities and Sciences, Hamilton, NJ (1-800-257-5126). The Big Chill: A Looming Ice Age? DVD OUR-337-02-KS; VHS OUR-337-02-KS; 2003, 50 minutes.

Films for the Humanities and Sciences, Hamilton, NJ (1-800-257-5126). Man and the Glaciers: Antarctic Researcher Claude Lorius. DVD OVW-401-58-KS; VHS OVW-401-58-A; 2008, 52 minutes.

Insight Media, New York, NY (800-233-9910). Glaciers. DVD #79-AS-866, 2004, 15 minutes.

Insight Media, New York, NY (800-233-9910). Continental Glaciation. Windows CD-ROM #79-AS-1376, 2005.

Select Reference Books: Benn, Douglas and David J. A. Evans. Glaciers and Glaciation, 2nd ed.London, UK: HodderArnold Publishing, 2010.

Bennett, Matthew R. and Neil F. Glasser (eds.). Glacial Geology: Ice Sheets and Landforms, 2nd ed. New York, NY: John Wiley and Sons, Inc., 2009.

Bradeye, Raymond S. Paleoclimatology: Reconstructing Climates of the Quaternary, 2nd ed.

Waltham, MA: Academic Press/Elsevier, 1999.

Oliver, John E. (ed.). Encyclopedia of World Climatology. Dordrecht, Netherlands: Springer, 2005.