Mcknights physical geography a landscape appreciation 12th edition hess solutions manual

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Chapter 7: Atmospheric Disturbances

Chapter 7: Atmospheric Disturbances Learning Outcomes Learning Outcome 7.1 Define air mass and list the three characteristics of an air mass. 176 Learning Outcome 7.2 Describe the origin of air masses in terms of source regions. 176 Learning Outcome 7.3 Explain how air masses are named. 176 Learning Outcome 7.4 Identify the names and properties of the six classes of air masses. 177 Learning Outcome 7.5 Identify the source regions and associated air masses that affect North America. 177 Learning Outcome 7.6 Define front and identify the four types of front. 178 Learning Outcome 7.7 Describe the formation of a cold front and the weather patterns that occur as a cold front passes a location. 179 Learning Outcome 7.8 Describe the formation of a warm front and the weather patterns that occur as a warm front passes a location. 179 Learning Outcome 7.9 Describe the formation of a stationary front and the weather patterns on either side of a front. 179 Learning Outcome 7.10 Describe the formation of an occluded cold front and the weather patterns that occur as an occluded front passes a location. 182 Learning Outcome 7.11 Identify the general characteristics of atmospheric disturbances. 179 Learning Outcome 7.12 Explain the characteristics of a midlatitude cyclone. 183 Learning Outcome 7.13 Identify the four kinds of movement that midlatitude cyclones undergo.

180 Learning Outcome 7.14 Explain the stages in the life cycle of a midlatitude cyclone. 183 Learning Outcome 7.15 Explain the typical weather pattern associated with the passage of a midlatitude cyclone with the center to the north of a location. 184 Learning Outcome 7.16 Explain the typical weather pattern associated with the passage of a midlatitude cyclone with the center to the south of a location. 184 Learning Outcome 7.17 Describe the occurrence and distribution of midlatitude cyclones. 184

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Learning Outcome 7.18 Define midlatitude anticyclone and explain the associated weather patterns. 186 Learning Outcome 7.19 Define easterly wave and describe the associated weather characteristics.

187 Learning Outcome 7.20 Identify the three categories of tropical disturbances. 187 Learning Outcome 7.21 Explain the characteristics of a hurricane. 187 Learning Outcome 7.22 Describe the structure of a hurricane. 188 Learning Outcome 7.23 Describe the movement and lifespan of a hurricane. 188 Learning Outcome 7.24 Identify areas where hurricanes originate and their common tracks. 189 Learning Outcome 7.25 Explain hazards associated with hurricanes and the types of damage they cause. 191 Learning Outcome 7.26 Describe the possible connections between hurricanes and climate change. 193 Learning Outcome 7.27 Explain the three stages in the development of a thunderstorm. 194 Learning Outcome 7.28 Explain the formation of a mesocyclone. 198 Learning Outcome 7.29 Explain the formation of tornadoes. 196 Learning Outcome 7.30 Explain the ways storms can be monitored to enable informed decisions about personal safety. 199 Chapter 7: Atmospheric Disturbances This chapter evaluates the creation and functioning of short-term atmospheric anomalies.

Specifically, the chapter looks at the formation of air masses and fronts. The formation of hurricanes, thunderstorms, and tornadoes are examined.

Teaching Tip Have students choose a prominent hurricane from recent years. Have them plot its course and show its development. Have them identify the pressure near the eye wall, its size, and its wind speed every 200 miles along its course. Have them identify the wind patterns driving the storm.

TOPICS The Impact of Storms on the Landscape Air Masses Characteristics 172 © 2017 Pearson Education, Inc.

Chapter 7: Atmospheric Disturbances

Origin Source Regions Classification Movement and Modification North American Air Masses Fronts Types of Fronts Cold Fronts Warm Fronts Stationary Fronts Occluded Fronts Air Masses, Fronts, and Major Atmospheric Disturbances Midlatitude Disturbances Tropical Disturbances Localized Severe Weather Midlatitude Cyclones Characteristics Formation of Fronts Sectors Clouds and Precipitation Movements Life Cycle Cyclogenesis Occlusion Conveyor Belt Model of Midlatitude Cyclones Weather Changes with the Passing of a Midlatitude Cyclone Occurrence and Distribution Midlatitude Anticyclones Characteristics Relationships of Cyclones and Anticyclones Easterly Waves Tropical Cyclones: Hurricanes Categories of Tropical Disturbances Named Storms Characteristics Eye of a Hurricane Origin Movement Hurricane Tracks Life Span Damage and Destruction Hurricane Strength Storm Surges Heavy Rain and Flooding Hurricane Katrina “Super Storm” Sandy 173 © 2017 Pearson Education, Inc.

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Hurricane Patricia Hurricanes and Climate Change Number of Hurricanes Intensity of Hurricanes Localized Severe Weather Thunderstorms Development Downbursts Lightning Thunder Tornadoes Funnel Clouds Tornado Formation Strength Waterspouts Severe Storm Watches and Warnings Focus: Conveyer Belt Model of Midlatitude Cyclones Global Environmental Change: Are Tornado Patterns Changing?

Focus: Weather Radar CHAPTER OUTLINE I. The Impact of Storms on the Landscape A. Storms are phenomena that are more limited than the broad-scale wind and pressure systems.

1. They are transient and temporary.

B. Storms involve the flow of air masses as well as a variety of atmospheric disturbances.

C. They have short-run and long-run impacts.

1. In some parts of the world they have major influence on weather, some on climate.

2. Long-run storms include both positive and negative impacts on a landscape. a) Positive: promote diversity in vegetative cover, increase size of lakes and ponds, and stimulate plant growth.

II. Air Masses A. Air mass—a large parcel of air that has relatively uniform properties in the horizontal dimension and moves as an entity. Such extensive bodies are distinct from one another and compose the troposphere.

B. Characteristics 1. An air mass must meet three requirements: a) Must be large (horizontal and vertical). b) Horizontal dimension must have uniform properties (temperature, humidity, and stability). c) Must be distinct from surrounding air, and when it moves it must retain that distinction (not be torn apart).

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C. Origin 1. Formation occurs if air remains over a uniform land or sea surface long enough to acquire uniform properties. a) Source Regions—parts of Earth’s surface that are particularly suited to generate air masses because they are: (1) Extensive (2) Physically uniform (3) Associated with air that is stationary or anticyclonic D. Classification 1. Because the source region determines the properties of air masses, it is the basis for classifying them.

2. Use a one- or two-letter code.

3. Table 7-1 provides a simplified classification of air masses, along with the properties associated with each.

E. Movement and Modification 1. Some air masses remain in the source region indefinitely.

2. Movement prompts structural change. a) Thermal modification—heating or cooling from below b) Dynamic modification—uplift, subsidence, convergence, turbulence c) Moisture modification—addition or subtraction of moisture 3. Moving air mass modifies the weather of the region it moves through.

F. North American Air Masses 1. Physical geography of the U.S. landscape plays a critical role in air mass interactions. a) No east–west mountains to block polar and tropical air flows, so they affect U.S. weather/climate. b) North–south mountain ranges in the west modify the movement, and therefore the characteristics, of Pacific air masses.

2. Maritime tropical (mT) air from the Atlantic Ocean and the Caribbean Sea/Gulf of Mexico strongly influences the climate east of the Rockies in the United States, southern Canada, and much of Mexico. a) Primary source of precipitation; also brings periods of uncomfortable humid heat in summer.

3. Continental tropical (cT) air has an insignificant influence on North America, except for bringing occasional heat waves and drought conditions to the southern Great Plains.

4. Equatorial (E) air affects North America only through hurricanes.

III. Fronts A. Front—a zone of discontinuity between unlike air masses where properties of air change rapidly.

1. Is narrow but three-dimensional.

2. Typically several kilometers wide (even tens of kilometers wide).

3. Functions as a barrier between two air masses, preventing their mingling except in this narrow transition zone.

4. Though all primary physical properties are involved in a front, temperature provides the most conspicuous difference.

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5. Fronts lean, which allows air masses to be uplifted and adiabatic cooling to take place. a) Some lean so much, they are closer to horizontal than vertical.

(1) Always slopes so that warmer air overlies cooler air.

6. Fronts move in association with the direction of the more active air mass, which displaces the less active.

B. Cold Fronts 1. Cold front—the leading edge of a cool air mass actively displacing a warm air mass. a) Brings cold air. b) Leads to the rapid lifting of warm air, which makes it unstable and thus results in blustery and violent weather along the front. c) Weather maps show ground-level position of a cold front (usually has a protruding “nose”); clouds and precipitation tend to be concentrated along and immediately behind the ground-level position.

C. Warm Fronts 1. Warm front—the leading edge of an advancing warm air mass. a) Brings warm air. b) Results in clouds and precipitation, usually broad, protracted, and gentle, without much convective activity. c) Unstable rising air can result in showery and even violent precipitation. d) Weather maps show the ground-level position of a warm front; precipitation usually falls ahead of this position.

D. Stationary Fronts 1. Stationary front—the common boundary between two air masses in a situation in which neither air mass displaces the other.

E. Occluded Fronts 1. Occluded front—a complex front formed when a cold front overtakes a warm front.

F. Air Masses, Fronts, and Major Atmospheric Disturbances G. Two types of disturbances: stormy and calm.

H. Both types have common characteristics.

1. Smaller than the components of general circulation, but extremely variable in size.

2. Migratory and transient.

3. Relatively brief in duration.

4. Produce characteristic and relatively predictable weather conditions.

5. Midlatitude Disturbances a) Many kinds of atmospheric disturbances are associated with the midlatitudes, which are the principal battleground for tropospheric phenomena. b) Midlatitude cyclones and midlatitude anticyclones are more significant because of size and prevalence.

6. Tropical Disturbances a) Low latitudes are characterized by monotony, with the same consistent weather. b) The only breaks in this pattern are provided by transient disturbances such as hurricanes.

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7. Localized Severe Weather a) Occurs in many parts of the world. b) Constitutes short-lived but severe weather phenomena such as thunderstorms and tornadoes.

IV. Midlatitude Cyclones A. Midlatitude cyclone—large migratory low-pressure system that occurs within the middle latitudes and moves generally with the westerlies; also called lows or wave cyclones, and depressions. a) Probably the most significant of all atmospheric disturbances. b) Basically responsible for most day-to-day weather changes. c) Bring precipitation to much of the world’s populated regions.

1. Characteristics a) A typical mature midlatitude cycle is 1600 kilometers (1000 miles) in diameter and has an oval shape. b) Patterns of isobars, fronts, and wind flow in the Southern Hemisphere are mirror images of those in the Northern Hemisphere.

(1) In the Northern Hemisphere: (a) Circulation pattern converges counterclockwise. (b) Wind-flow pattern attracts cool air from the north and warm air from the south and creates two fronts. (i) These two fronts divide the cyclone into a cool sector north and west of the center and a warm sector south and east. (a) Size of sectors varies with location: on the ground, the cool sector is larger, but in the atmosphere, the warm sector is more extensive. (ii) Warm air rises along both fronts, causing cloudiness and precipitation, which follows patterns of cold and warm fronts. (iii)Much of the cool sector is typified by clear, cold, stable air, whereas air of the warm sector is often moist and tends toward instability, so the cool sector may have sporadic thunderstorms. May have squall fronts of intense thunderstorms.

B. Weather Changes With a Passing Front 1. With the passage of a cold front, the following changes typically occur: a) The temperature decreases sharply. b) Winds shift from southerly ahead of the front to northwesterly following it (in the Northern Hemisphere). c) The pressure falls as the front approaches and then rises after it passes. d) Generally clear skies are replaced by cloudiness and precipitation at the front.

C. Movements 1. Midlatitude cyclones move throughout their existence.

2. Average rate of movement is 30–45 kilometers per hour (20–30 miles per hour).

3. Cyclonic wind circulation of these midlatitude cyclones have winds that generally converge counterclockwise (in the Northern Hemisphere) into the center of the storm from all sides.

4. The cold front generally moves faster than the storm’s center.

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D. Life Cycle 1. Cyclogenesis a) Origin to maturity typically takes 3 to 6 days, then another 3 to 6 days to dissipate. b) Cyclogenesis—birth of cyclones. c) Most common cause believed to be upper-air conditions in the vicinity of the polar-front jet stream. d) Most begin as waves along the polar front. e) Cyclogenesis can also occur on the leeward side of mountains. f) Often bring heavy rain or snowstorms to the northeastern United States and southeastern Canada.

2. Occlusion a) After cyclonic circulation is well developed, occlusion begins. b) After an occluded front is fully developed, the cyclone dissipates.

3. Conveyer Belt Model of Midlatitude Cyclones a) Model first presented by meteorologists in Norway in the 1920s. b) Conveyer belt model offers a better explanation.

E. Weather Changes With the Passing of a Midlatitude Cyclone 1. Temperature a) As a cold front passes, temperature drops abruptly.

2. Pressure a) Pressure falls as the front approaches, and as the front passes the pressure rises steadily.

3. Wind a) Winds in the warm sector come from the south. Once the front passes, winds shift and come from the west or northwest.

4. Clouds and Precipitation a) As a cold front approaches, clear skies are replaced by cloudiness and precipitation. b) After the front passes, the conditions clear.

5. Occurrence and Distribution a) Occur at scattered but irregular intervals throughout the zone of the westerlies. b) Route of a cyclone is likely to be undulating and erratic, but it generally moves west to east.

VI. Midlatitude Anticyclones A. Midlatitude anticyclone—an extensive migratory high-pressure cell of the midlatitudes that moves generally with the westerlies.

B. Characteristics a) Typically larger than a midlatitude cyclone, but also moves west to east. b) Travels at the same rate, or a little slower, than a midlatitude cyclone.

(1) Is prone to stagnate or remain over the same region (while cyclones do not). (a) Can cause a concentration of air pollutants.

C. Relationships of Cyclones and Anticyclones 1. Cyclones and anticyclones alternate with one another in an irregular sequence. a) Often a functional relationship between the two.

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(1) Can visualize an anticyclone as a polar air mass with the cold front of a cyclone as its leading edge.

VII. Easterly Waves A. Easterly wave—a long but weak migratory low-pressure system in the tropics between 5° and 30° of latitude.

B. They are usually several hundred kilometers long and nearly always oriented north– south.

C. They drift westward on the trade winds.

D. Convergent conditions behind the wave generate thunderstorms and cloudiness.

1. Sometimes intensify into hurricanes.

2. Bring characteristic weather of the trade winds with them.

VIII. Tropical Cyclones: Hurricanes A. Tropical cyclone—a storm most significantly affecting the tropics and subtropics, which is intense, revolving, rain-drenched, migratory, destructive, and erratic. Such a storm system consists of a prominent low-pressure center that is essentially circular in shape and has a steep pressure gradient outward from the center. a) Tropical cyclones provide the only break in weather in low latitudes. b) Also called: (1) Hurricanes in North and Central America (2) Typhoons in the western North Pacific (3) Baguios in the Philippines (4) Tropical cyclones in the Indian Ocean and Australia 2. With diameters of between 160 and 1000 kilometers, tropical cyclones are smaller than midlatitude cyclones.

3. Three categories of tropical cyclones: a) Tropical depression—winds of 33 knots (61 kilometers, or 38 miles, per hour) or less. b) Tropical storm—winds between 34 and 63 knots (63 and 118 kilometers, or 39 and 73 miles, per hour) c) Hurricane—winds of 64 knots (119 kilometers, or 74 miles, per hour) or more; can double and even triple that minimum.

4. World Meteorological Society (WMO) is responsible for monitoring tropical storms globally. a) Several local warning centers operate regionally: (1) National Hurricane Center (Miami, FL) monitors the North Atlantic and northeastern Pacific.

(2) Central Pacific Hurricane Center (Hawaii) monitors the north-central Pacific.

(3) Japan Meteorological Agency (Japan) monitors the northwestern Pacific.

B. Characteristics 1. A hurricane pulls in warm, moist air for fuel, and this air rises and cools adiabatically. a) This causes condensation and in turn releases heat, which further increases the instability of the air. b) Not characterized as midlatitude cyclones.

(1) Dissimilar air masses are not pulled together.

(2) All air in a hurricane is warm and moist.

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2. Eye of a Hurricane a) Eye—the nonstormy center of a tropical cyclone, which has a diameter of 16–40 kilometers (10–25 miles). In the eye there are no updrafts, but instead a downdraft that inhibits cloud formation. b) Eyewall—peripheral zone at the edge of the eye where winds reach their highest speed and where updrafts are most prominent. c) Weather pattern within a hurricane is symmetrical. d) Comprised of bands of dense cumulus and cumulonimbus clouds called spiral rain bands.

3. Eyewall replacement—the process in which a new wall of storms surrounds the wall of storms circling the hurricane’s eye. When this occurs, the inner wall disintegrates so the new wall replaces it. This process tends to weaken the storm.

C. Origin 1. Form only over warm oceans and where there is no significant wind shear.

2. The Coriolis effect plays key role: it’s at a minimum at the equator, and no hurricane has been observed to form within 3° of the equator or cross over it. a) Rare to have a hurricane closer than 8° to 10° from equator.

3. The exact mechanism of formation is not clear, but they always grow from some preexisting disturbance.

4. Movement a) Most common in the North Pacific basin (origination in the Philippines and west of southern Mexico and Central America).

(1) West central portion of the North Atlantic basin, extending into the Caribbean, and Gulf of Mexico is third in prevalence.

(2) Totally absent from the South Atlantic and from the southeastern part of the Pacific. (a) Absent apparently because the water is too cold and because high pressure dominates. b) General pattern of movement is highly predictable.

(1) About one-third travel east to west without much latitudinal change.

(2) About two-thirds start off on an east–west path and then curve poleward. (a) Exception occurs in the southwestern Pacific Ocean north and northeast of New Zealand, where the general circulation pattern steers hurricanes, so they travel west to east.

(3) Average hurricane lasts a week; those that remain over tropical oceans can live up to four weeks. (a) Dies down over continents because energy source of warm, moist air is cut off. (b) Dies down in midlatitudes because of cooler environment. (i) In midlatitudes, can diminish in intensity but grow in size and become a midlatitude cyclone.

D. Damage and Destruction 1. High seas, or a storm surge, cause the most damage.

2. Storm size is key to how much damage is caused, then physical configuration of landscape and population size and density of affected area.

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3. Saffir–Simpson Hurricane Scale has been established to rank the intensity of hurricanes. a) Ranges from 1 to 5, with 5 being the most severe (Table 7-2).

4. Heavy Rain and Flooding a) Strong hurricanes can inflict heavy damage from flooding.

(1) In 2011, Hurricane Irene brought extensive flooding to the northeastern United States and southeastern Canada.

E. Hurricanes and Climate Change 1. The 2005 hurricane season in the North Atlantic was the most active on record. a) Included 28 named storms. b) Three of the most powerful hurricanes measured in terms of minimum atmospheric pressure in the eye.

2. The 2010 and 2011 seasons tied for third overall, with 19 named storms.

3. Connection between ocean temperature and hurricane formation and connection between global warming and ocean temperature generates question: a) Will global warming increase hurricane activity?

4. There has been a general increase in the annual number of hurricanes in the North Atlantic in the past 25 years. a) Some meteorologists attribute the increase to a multidecadal cycle of hurricane activity that has been well documented since the early 1990s (the Multi-Decadal Signal).

(1) Combination of high sea-surface temperatures (SSTs), low wind shear, and expanded upper-level westward flow of the atmosphere off North Africa. b) Underlying components of the Multi-Decadal Signal are not completely understood, but the recent increase in hurricane frequency can likely be explained without tying it to global warming. c) However, hurricane intensity may be tied to global warming.

(1) Reflected in the potential relationship between higher SSTs and hurricane intensity. d) The 2013–2014 Fifth Assessment Report of the Intergovernmental Panel on Climate Change concluded that it was virtually certain (99–100 percent probability) that we will observe an increase in tropical cyclones during the century as a result of increasing global temperatures.

IX. Localized Severe Weather A. Occur on a more localized scale than do tropical and midlatitude cyclones and anticyclones.

B. Thunderstorms 1. Thunderstorm—violent convective storm accompanied by thunder and lightning, usually localized and short lived. a) Vertical air motion, considerable humidity, and instability combine to create towering cumulonimbus clouds, so thunderstorms are always associated with this combination. b) Frequently occur in conjunction with other kinds of storms.

(1) For example, hurricanes, tornadoes, fronts (especially cold fronts), midlatitude cyclones, and orographic lifting.

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c) Associated with other mechanisms that can trigger unstable uplift.

(1) Mechanism triggers uplift of warm, moist air.

(2) Cumulus stage—updrafts prevail and clouds grow. Rise to above freezing level, where supercooled water droplets and ice crystals coalesce, then fall.

Initiate a downdraft.

(3) Mature state—updrafts and downdrafts coexist as the cloud continues to enlarge (but precipitation is leaving the bottom of the cloud). Most active time.

(4) Dissipating state—downdrafts dominate and turbulence ceases. d) Virtually unknown poleward of 60˚ of latitude.

C. Lightning 1. More than 8.5 million lightning bolts occur daily worldwide.

2. Most frequently, lightning occurs as exchanges between adjacent clouds or between the upper and lower portions of the same cloud; it also occurs as an electrical connection of ionized air from cloud to ground.

3. The sequence that leads to lightning discharge is known, but the mechanism for electrification is not. a) Sequence: (1) Large cumulonimbus cloud experiences a separation of electrical charges.

(2) Positively charged particles are mostly high in the cloud, while negatively charged particles tend to concentrate at the base.

(3) Growing negative charge in the base attracts a growing positive charge on Earth’s surface immediately below the cloud.

(4) An insulating barrier lies between the cloud base and surface. (a) Contrast between the cloud base and surface builds to tens of millions of volts and overcomes the insulating barrier. (b) A finger of negative current flicks down from the cloud and meets a positive charge darting upward from the ground, causing lightning.

4. Cause is unknown; different hypotheses. a) Most popular hypothesis: updrafts carry positively charged particles to the top, while falling ice pellets gather negative charges and transport them downward.

5. Thunder—an instantaneous expansion of air caused by the abrupt heating that a lightning bolt produces. This expansion creates a shock wave that becomes a sound wave. a) Can time the distance that lightning is away because of the different rates thunder and lightning travel at (speed of sound vs. speed of light).

(1) Five-second interval equals about a mile; three-second interval equals about a kilometer.

D. Tornadoes 1. Tornado—a localized cyclonic low-pressure cell surrounded by a whirling cylinder of wind spinning so violently that a partial vacuum develops within the funnel. a) Has the most extreme pressure gradients known (as much as a 100-millibar difference between the tornado center and the air immediately outside the funnel).

(1) Extreme pressure difference produces winds of extraordinary speed. (a) How fast are winds?

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(i) No one knows because tornadoes blow to bits anemometers (instrument for measuring speed). Maximum estimates range from 320 to 800 kilometers (200 to 500 miles) per hour. (b) Greatest damage tends to be from a combination of strong winds, flying debris, and extreme updrafts. (i) Old advice of opening windows during tornado event is no longer recommended because it may increase chance of injury from flying debris.

2. Tornado Formation a) Exact mechanism of formation is unknown.

(1) Usually develops in warm, moist, unstable air associated with midlatitude cyclones.

(2) High wind shear (horizontally rotating air) may cause strong updrafts to form in a supercell thunderstorm. (a) The rotating air may then be tilted vertically, forming a mesocyclone. (i) About 50 percent of all mesocyclones formed result in tornadoes.

(3) Most often develops along a squall line that precedes a rapidly advancing cold front, or along the cold front.

(4) Spring and early summer are favorable for development because there’s considerable air-mass contrast present in the midlatitudes at that time.

(5) Most occur in midafternoon, at time of maximum heating.

(6) More than 90 percent of all reported tornadoes occur in the United States. (a) Reflects optimum environmental conditions. (i) Relatively flat terrain of the central and southeastern United States provides uninhibited interaction of Canadian cP and Gulf mT air masses.

3. Strength a) The strength of a tornado is described using the Enhanced Fujita Scale (EF) (Table 7-3).

(1) Scale is based on estimates of 3-second gust wind speeds.

(2) Percentage of U.S. tornadoes that fall within the five categories are: (a) EF 0–1 (light or moderate): 69 percent (b) EF 2–3 (strong or severe): 29 percent (c) EF 4–5 (devastating or incredible): 2 percent (3) Annual U.S. tornado death toll has decreased because of better forecasting.

(4) Waterspouts occur over ocean; have less pressure gradient, gentler winds, and reduced destructive capability.

4. Severe Storm Watches and Warnings a) Storm watch is an advisory issued for a region where, over the next 4 to 6 hours, the conditions are favorable for the development of severe weather. b) Storm warning is issued by a local weather forecasting office when a severe thunderstorm or tornado has actually been observed.

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X. Focus: Conveyer Belt Model of Midlatitude Cyclones A. Satellite and weather balloon measurements have revealed that midlatitude cyclones involve more than just surface fronts and a low-pressure center; they also tend to include several well-defined channels of air called “conveyor belts” (Figure 7-A).

B. The Warm Conveyor Belt 1. The midlatitude cyclone’s surface low draws air northward from the southeastern portion of the cyclone. The air to the southeast tends to be warm and moist because those are characteristics of the air mass where it originates.

2. A warm conveyor belt develops from this air, which starts at the surface, but because it is less dense, it eventually rises up and over the cooler air to the north of the warm front.

3. This can bring moisture and even snow as it contributes moisture to the cold conveyor belt.

C. The Cold Conveyor Belt 1. Just north of the warm front, cooler, drier surface air moves westward toward the cyclone’s central low, forming the cold conveyor belt.

2. Like the warm conveyor belt, some of this cold air can rise to merge with the general westerly flow at upper levels. a) However, the cold conveyor belt can also split, with the rest of the air turning cyclonically and rising as it moves toward the low-pressure center. b) In winter, this often produces the midlatitude cyclone’s heaviest snowfall just northwest of the low.

D. The Dry Conveyor Belt 1. On the western side of a typical midlatitude cyclone, convergence in the upper troposphere produces descending air, some of which swirls counterclockwise into the cyclone’s low, forming the dry conveyor belt.

2. This air from the upper troposphere is much drier than the air in the other conveyor belts because it is farther from the surface and thus farther from sources of moisture.

3. Few clouds, if any, can form in this dry air, which often produces a “dry slot” of air just behind the cold front that is lacking in clouds compared with other areas nearby. a) The dry conveyor belt gives the cyclone a “comma” shape by separating clouds defining the comma’s head, formed primarily by the cold conveyor belt, from clouds defining the comma’s tail, formed primarily by the warm conveyor belt.

XI. Global Environmental Change: Are Tornado Patterns Changing?

A. The most active part of 2011 was from April 25 to April 28.

1. Produced 343 confirmed tornadoes and killed 321 people.

2. Four of these tornadoes produced damage up to the EF-5 category—the first EF-5 tornadoes anywhere on Earth since 2008.

B. Post-storm damage surveys are the main evidence of tornadoes and their strengths, though storm spotters and the public provide information as well.

1. Doppler weather radar alone does not confirm tornadoes, but the U.S. Doppler network, installed in the 1990s, and advances since that deployment have improved warnings and detection.

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2. The number of recorded tornadoes has increased, but to see whether the actual number of tornadoes has increased, researchers tend to analyze trends in larger tornadoes.

3. The overall number of large tornadoes hasn’t changed significantly, but outbreaks such as the one in 2011 are becoming more common. a) Outbreaks don’t happen every year, but when they do, they’re deadly—by the end of 2011 tornadoes had killed 551 people in the United States, the largest annual total in 66 years of modern records.

C. Thunderstorms tend to be more frequent and powerful when the surface is hot and humid.

1. Global climate change is simultaneously increasing surface temperatures, increasing evaporation rates, and decreasing the number of days with adequate wind shear for tornadic formation.

2. Observed changes, which also fit global climate changes, in tornado patterns are fewer days with tornadoes but on days when there is enough wind shear, increased tendency for outbreaks.

XII. Focus: Weather Radar A. NEXRAD (next-generation radar) 1. NEXRAD uses the Doppler effect to determine the severity of a storm. a) Doppler effect—an apparent change in sound and electromagnetic waves that occurs because a source is moving; the apparent change depends on whether the source is approaching or receding.

(1) NEXRAD transmits microwaves through the atmosphere and then measures how much of this transmitted signal is reflected to give an estimate of the storm’s intensity. b) More precise than radar.

2. NEXRAD underwent hardware and software upgrades to incorporate dual polarization. a) This simultaneously emits pulses of horizontally oriented and vertically oriented microwaves that are reflected in a manner analogous to the radar pulses. b) Based on the reflected microwave data, the software identifies the forms of precipitation present and the precipitation rate. c) An important use of NEXRAD is in detecting tornadoes. d) Tornado funnels are usually only a few hundred meters across, and it is rare that NEXRAD can detect them using reflectivity alone. e) NEXRAD can detect motion to within 0.9 meters per second (2 miles per hour).

McKnight and Hess 12e Chapter 7 Learning Checks Learning Check 7-1 How do air masses form? Why do air masses rarely originate in the midlatitudes?

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parts of Earth’s surface that are particularly suited to generate air masses because they are extensive, physically uniform, and are associated with air that is stationary or anticyclonic.

In the band of the westerlies (the midlatitudes), the atmosphere is rarely quiet long enough for an air mass to form.

Learning Check 7-2 What are the temperature and moisture characteristics of a maritime polar (mP) air mass? A continental tropical (cT) air mass? Explain.

Maritime polar air masses are moist and cold because they originate over bodies of water at high latitudes.

Continental tropic air masses are hot and dry, and originate over northern Mexico and the southwestern United States.

Learning Check 7-3 Compare the characteristics of cold fronts and warm fronts.

The primary difference between a warm front and a cold front is the temperature of the air they bring: warm fronts bring warm air, whereas cold fronts bring cold air. The average cold front is twice as steep as the average warm front. Cold fronts normally move faster than warm fronts.

This combination of steeper slope and faster advance in cold fronts leads to a rapid lifting of any warm air in front of a cold front. As such, cold fronts result in blustery and violent weather, compared with the more gentle precipitation commonly associated with warm fronts (they have a very gradual frontal uplift of warm air, so clouds form slowly without much turbulence). But while precipitation with a cold front is usually of a higher intensity than that with a warm front, it is also of a shorter duration than the precipitation in a warm front. Finally, precipitation usually falls ahead of the ground-level position of the warm front, whereas it tends to concentrate along and immediately behind the cold front’s ground-level position.

Learning Check 7-4 What causes fronts to develop within a midlatitude cyclone?

A typical mature midlatitude cycle is 1600 kilometers (1000 miles) in diameter and has an oval shape.

In the Northern Hemisphere, the circulation pattern converges counterclockwise; the wind-flow pattern attracts cool air from the north and warm air from the south, which creates two fronts.

These two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. The size of sectors varies with location: on the ground, the cool sector is larger, but in the atmosphere, the warm sector is more extensive. Warm air rises along both fronts, causing cloudiness and precipitation, which follows patterns of cold and warm fronts. Much of the cool sector is typified by clear, cold, stable air, whereas air of the warm sector is often moist 186 © 2017 Pearson Education, Inc.

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and tends toward instability, so may have sporadic thunderstorms and may have squall fronts of intense thunderstorms.

Patterns of isobars, fronts, and wind flow in the Southern Hemisphere are mirror images of those in the Northern Hemisphere.

Learning Check 7-5 Explain where and why precipitation develops within a midlatitude cyclone.

See Learning Check question 7-4.

Learning Check 7-6 Describe the process that forms an occluded front.

Because cold fronts are denser, they travel at a higher velocity than warm fronts. Occlusion is the process where a cold front overtakes a warm front and then displaces the warm air sector between them. Nearly all midlatitude cyclones experience occlusion.

Learning Check 7-7 Why is pressure falling as a cold front approaches, and rising as a cold front moves away?

As a cold front approaches, the cold, dense air of the cold front displaces the relatively warm, moist air of the warm-air sector. This causes the portion of the warm-air sector adjacent to the cold front to be lifted, creating lower atmospheric pressure. As the cold front passes, cold dense air dominates, causing the barometric pressure to rise again.

Learning Check 7-8 Why are midlatitude anticyclones associated with dry weather?

High atmospheric pressure predominates with midlatitude anticyclones. The overall weather associated with a midlatitude cyclone is clear and dry, with little or no opportunity for cloud formation because there is no rising air in a midlatitude cyclone and no adiabatic cooling.

Learning Check 7-9 Describe the characteristics of an easterly wave.

An easterly wave is a long but weak migratory low-pressure system in the tropics between 5° and 30° of latitude. They are usually several hundred kilometers long and nearly always oriented north–south, and they tend to drift westward on the trade winds. Convergent conditions behind the wave generate thunderstorms and cloudiness. These easterly waves sometimes intensify into hurricanes. They also tend to bring characteristic weather of the trade winds with them.

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Learning Check 7-10 Why is warm, moist air the “fuel” for a hurricane?

Hurricanes form only over warm oceans in the tropics. The ocean water temperature generally needs to be at least 26.5°C to a depth of 50 meters. There also cannot be any significant wind shear. The Coriolis effect plays a key role: it’s at a minimum at the equator, and no hurricane has been observed to form within 3° of the equator or cross over it. It is rare to have a hurricane closer than 8° to 10° from the equator. The exact mechanism of formation is not clear, but they always grow from some preexisting disturbance.

Learning Check 7-11 Why can hurricanes move up into the midlatitudes along the East Coast of North America, but not along the West Coast?

Hurricanes sometimes survive (with diminished intensity) off the east coast of continents in the midlatitudes because the warm ocean currents there contribute energy to the cyclones; hurricanes do not survive in the midlatitudes off the west coasts of continents because of the cool ocean currents there.

Learning Check 7-12 What causes a hurricane’s storm surge?

A storm surge is a surge of wind-driven water caused by a hurricane. It is as much as 8 meters (25 feet) above normal tidal level. These surges occur when a hurricane pounds into a shoreline.

These surges are also accentuated by the reduced atmospheric pressure of the hurricane.

Learning Check 7-13 Describe the sequence of development and dissipation of a typical thunderstorm.

A thunderstorm is a violent convective storm accompanied by thunder and lightning; it is usually localized and short lived. Vertical air motion, considerable humidity, and instability combine to create towering cumulonimbus clouds, so thunderstorms are always associated with this combination.

Thunderstorms are associated with other mechanisms that can trigger unstable uplift. The mechanism triggers uplift of warm, moist air.

During the cumulus stage, updrafts prevail and clouds grow. The clouds rise to above freezing level, where supercooled water droplets and ice crystals coalesce, then fall. This then initiates a downdraft.

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During the mature state, updrafts and downdrafts coexist as the cloud continues to enlarge (but precipitation is leaving the bottom of the cloud). This is the most active time.

During the dissipating state, downdrafts dominate and turbulence ceases.

Learning Check 7-14 Explain the sequence of formation of a typical tornado associated with a mesocyclone.

The exact mechanism of formation is unknown. They usually develop in the warm, moist, unstable air associated with a midlatitude cyclone. High wind shear (horizontally rotating air) may cause strong updrafts to form in a supercell thunderstorm. The rotating air may then be tilted vertically, forming a mesocyclone. About 50 percent of all mesocyclones result in tornadoes. These most often develop along a squall line that preceded a rapidly advancing cold front, or along the cold front. Spring and early summer are favorable for development because there’s considerable air-mass contrast present in the midlatitudes at that time. Likewise, most occur in midafternoon, at the time of maximum heating. There is also a region bias in that more than 90 percent of all reported tornadoes occur in the United States. The regionality of this phenomenon is a result of optimum environmental conditions: The relatively flat terrain of central and southeastern United States provides uninhibited interaction of Canadian cP and Gulf mT air masses.

Chapter 7 Learning Review Key Terms and Concepts Air Masses (p. 176) 1. What is an air mass, and what conditions are necessary for one to form?

The formation of air masses occurs if air remains over a uniform land or sea surface long enough to acquire uniform properties. Air masses form in distinctive source regions. These regions are parts of Earth’s surface that are particularly suited to generate air masses because they are extensive, physically uniform, and are associated with air that is stationary or anticyclonic.

2. What regions of Earth are least likely to produce air masses? Why? Mountainous areas, areas that vary in altitude and/or covering, and areas where air above moves or is cyclonic are least likely to produce air masses. To develop, air masses need an extensive uniform land or sea surface that allows air to stagnate and develop homogeneous physical characteristics. Ideal source regions are ocean surfaces and extensive flat land areas that have a uniform covering of snow, forest, or desert.

3. Contrast and explain the moisture and temperature characteristics of a mT (maritime tropical) air mass with that of a cP (continental polar) air mass.

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Maritime tropical air masses are warm and moist because they originate within tropical latitudes and over bodies of water. Continental polar air masses are cold and dry because they originate over high-latitude landmasses.

Fronts (p. 178) 4. What is the relationship of air masses to a front?

When unlike air masses meet, they do not mix readily; instead, a boundary zone (i.e., a front) develops between them.

5. What is a cold front? What is a warm front?

The primary difference between a warm front and a cold front is the temperature of the air they bring: warm fronts bring warm air, whereas cold fronts bring cold air. The average cold front is twice as steep as the average warm front. Cold fronts normally move faster than warm fronts.

This combination of steeper slope and faster advance in cold fronts leads to a rapid lifting of any warm air in front of a cold front. As such, cold fronts result in blustery and violent weather, compared with the more gentle precipitation commonly associated with warm fronts (they have a very gradual frontal uplift of warm air, so clouds form slowly, without much turbulence). But while precipitation with a cold front is usually of a higher intensity than that of a warm front, it is also of a shorter duration than the precipitation in a warm front. Finally, precipitation usually falls ahead of the ground-level position of the warm front, whereas it tends to concentrate along and immediately behind the cold front’s ground-level position.

6. What is a stationary front?

A stationary front is a common boundary that develops when two air masses meet but neither displaces the other.

Midlatitude Cyclones (p. 180) 7. Describe the pressure and wind patterns of a midlatitude cyclone.

A typical mature midlatitude cycle is 1600 kilometers (1000 miles) in diameter and has an oval shape.

In the Northern Hemisphere, the circulation pattern converges counterclockwise; the wind-flow pattern attracts cool air from the north and warm air from the south, which creates two fronts.

These two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. The size of sectors varies with location: on the ground, the cool sector is larger, but in the atmosphere, the warm sector is more extensive. Warm air rises along both fronts, causing cloudiness and precipitation, which follows patterns of cold and warm fronts. Much of the cool sector is typified by clear, cold, stable air, whereas air of the warm sector is often moist and tends toward instability, so may have sporadic thunderstorms and may have squall fronts of intense thunderstorms.

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8. Describe the locations of fronts and the surface “sectors” of a mature midlatitude cyclone.

See question 7.

9. Describe and explain the regions of cloud development and precipitation within a midlatitude cyclone.

See question 7.

10. Discuss the four components of movement of a midlatitude cyclone.

Four kinds of movements are involved: 1) Whole system traverses from west to east as a major disturbance in the westerlies.

Rate averages 32 to 48 kilometers (20 to 30 miles) per hour, although it moves faster in winter than in summer.

2) Whole system has a cyclonic wind circulation, with direction dependent on the hemisphere.

3) The cold front normally moves faster than the storm, so it moves into and displaces the warm fronts.

4) The warm front usually advances more slowly than the storm.

11. Explain the process of occlusion.

Occlusion is the process where a cold front overtakes a warm front and then displaces the warm air sector between them.

12. Why does an occluded front usually indicate the “death” of a midlatitude cyclone?

Occluded fronts are caused by the more rapid movement of cold fronts compared with warm fronts in a midlatitude cyclone. Since the typical pattern in a midlatitude cyclone is a cold front preceded by a warm front, the cold front eventually overtakes the warm front and effectively zips up the midlatitude cyclone. Once occlusion in a midlatitude cyclone begins, the warm air separating the cold and warm fronts begins to become pinched away from the surface region of the midlatitude cyclone and forced aloft. This creates a temperature inversion situation, where a layer of warmer air overlies a layer of cold air. This temperature inversion impedes atmospheric uplift and generates atmospheric stability. This then leads to the dissipation of the midlatitude cyclone.

13. Discuss the cyclogenesis of midlatitude cyclones. What is the relationship between upperlevel airflow and the formation of surface disturbances in the midlatitudes?

There seems to be a close relationship between upper-level airflow and ground-level disturbances. When winds aloft meander north to south, in a motion called meridional airflow, cyclonic activity at the ground level is intensified. With this meridional airflow, large waves of alternating-pressure troughs and ridges are formed, which contribute to the surface disturbances.

This does not usually occur with west–east upper airflow. For a cyclone to develop at ground level, there must be both a convergence of air near the ground and a divergence in the air that’s 191 © 2017 Pearson Education, Inc.

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aloft. The divergence can be caused by changes in the speed or direction of wind flow, but it usually involves north-to-south meanders in the Rossby waves and the jet stream.

14. Describe and explain the changes in wind direction, atmospheric pressure, sky conditions (such as clouds and precipitation), and temperature with the passing of a cold front of a midlatitude cyclone.

As a cold front approaches, large, vertically developed clouds (cumulonimbus) appear in the direction the front is originating. As the surface portion of the front passes, there are vigorous downpours and thunderstorms from cumulonimbus clouds. Because of higher pressure behind the front and the steepness of the front, the precipitation is usually of a short but intense duration.

Clouds behind the front, if any, tend to be limited in vertical thickness and usually do not produce precipitation.

As the surface portion of the cold front passes, there is a temperature drop. This is because the air behind the cold front originated as a continental polar air mass.

As the front approaches and passes, barometric pressure rapidly rises, indicating that clearing conditions should occur within 12 to 24 hours.

Midlatitude Anticyclones (p. 186) 15. Describe the pressure pattern, wind direction, and general weather associated with a midlatitude anticyclone.

High atmospheric pressure predominates with midlatitude anticyclones. The weather associated with a midlatitude cyclone is clear and dry, with little or no opportunity for cloud formation. At the center, wind movement is very limited, but it increases progressively outward, so that at the margins, and particularly the eastern margin, there may be strong winds. Winter anticyclones have very low temperatures.

16. How are midlatitude anticyclones often associated with midlatitude cyclones?

Midlatitude cyclones and anticyclones alternate with one another in an irregular sequence. There is often a functional relationship between the two. This can be visualized with an anticyclone being a polar air mass with the cold front of cyclone as its leading edge.

Easterly Waves (p. 187) 17. What is an easterly wave?

An easterly wave is a long but weak migratory low-pressure system in the tropics between 5° and 30° of latitude. They are usually several hundred kilometers long and nearly always oriented north–south, and they tend to drift westward on the trade winds. Convergent conditions behind the wave generate thunderstorms and cloudiness. These easterly waves sometimes intensify into hurricanes. They also tend to bring characteristic weather of the trade winds with them.

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Tropical Cyclones: Hurricanes (p. 187) 18. Distinguish among a tropical depression, a tropical storm, and a hurricane.

A tropical depression has wind speeds less than 33 knots but develops a closed wind circulation pattern.

A tropical storm has wind speeds between 34 and 63 knots.

A hurricane has winds greater than 64 knots.

19. Describe and explain the pressure and wind patterns of a tropical cyclone (hurricane).

Hurricanes are enormous cyclones that possess prominent low-pressure centers that are essentially circular, with a steep pressure gradient outward from the center. As a result, strong winds spiral inward and ascend in rapid updrafts. Near the hurricane’s center there is also a downdraft that inhibits cloud formation (the hurricane’s eye). The weather pattern within the hurricane is symmetrical around the eye, with winds and bands of dense cumulus and cumulonimbus clouds spiraling inward from the edge of the storm to the eye. These clouds produce heavy rains that generally increase in intensity, until within the eye, where there is no rain and no low clouds; in the eye, scattered high clouds may part to let in intermittent sunlight.

20. Discuss the characteristics of the eye of a hurricane.

The eye of a hurricane is the nonstormy center of a tropical cyclone, which has a diameter of 16 to 40 kilometers (10 to 25 miles). In the eye, there are no updrafts, but instead a downdraft that inhibits cloud formation. The eye wall is a peripheral zone at the edge of the eye where winds reach their highest speed and where updrafts are most prominent.

21. What is wind shear?

Wind shear refers to the significant change in wind direction or wind speed with increasing elevation.

22. Discuss the conditions necessary for a hurricane to form.

Hurricanes form only over warm oceans and where there is no significant wind shear. The Coriolis effect plays a key role: it’s at a minimum at the equator, and no hurricane has been observed to form within 3° of the equator or cross over it. It is rare to have a hurricane closer than 8° to 10° from the equator. The exact mechanism of formation is not clear, but they always grow from some preexisting disturbance.

23. Describe and explain the typical paths taken by hurricanes in the North Atlantic Ocean basin.

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without much latitudinal change. About two-thirds start off on an east–west path and then curve poleward. Here they either dissipate over the adjacent continent or become enmeshed in the general flow of the midlatitude westerlies.

24. Why do hurricanes weaken when they move over land?

The average hurricane lasts a week; however, those that remain over tropical oceans can live up to 4 weeks. Hurricanes die down over continents because the energy source of warm, moist air is cut off. Hurricanes likewise die down in midlatitudes because of the cooler environment.

25. Briefly explain the Saffir–Simpson Hurricane Scale.

The Saffir–Simpson Hurricane Scale ranks the relative intensity of hurricanes based on eye pressure, wind speed, and storm-surge height. The scale ranges from 1 to 5, with 5 being the most severe.

26. What is a hurricane storm surge, and what causes one?

A storm surge is a surge of wind-driven water caused by a hurricane. It is as much as 8 meters (25 feet) above the normal tidal level. These surges occur when a hurricane pounds into a shoreline. These surges are also accentuated by the reduced atmospheric pressure of the hurricane.

27. Do hurricanes have any beneficial effects? Explain.

Several regions depend on hurricanes for much of their water supply: northwestern Mexico, northern Australia, and southeastern Asia. Even in other areas, hurricane-induced rainfall is often a critical source of moisture for agriculture: though a hurricane’s winds and flooding may destroy crops within its immediate path, the hurricane’s rains nurture a much more extensive area.

Localized Severe Weather (p. 194) 28. Discuss the general sequence of thunderstorm development and dissipation.

A thunderstorm is a violent convective storm accompanied by thunder and lightning; it is usually localized and short lived. Vertical air motion, considerable humidity, and instability combine to create towering cumulonimbus clouds, so thunderstorms are always associated with this combination.

Thunderstorms are associated with other mechanisms that can trigger unstable uplift. The mechanism triggers the uplift of warm, moist air.

During the cumulus stage, updrafts prevail and clouds grow. The clouds rise to above freezing level, where supercooled water droplets and ice crystals coalesce, then fall. This then initiates a downdraft.

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During the mature state, updrafts and downdrafts coexist as the cloud continues to enlarge (but precipitation is leaving the bottom of the cloud). This is the most active time.

During the dissipating state, downdrafts dominate and turbulence ceases.

29. What is the relationship of thunder to lightning?

The abrupt heating of the localized atmosphere from a lightning bolt causes an instantaneous expansion of the air, which causes a shock wave that we hear as thunder.

30. Describe the wind and pressure characteristics of a tornado.

A tornado is a localized cyclonic low-pressure cell surrounded by a whirling cylinder of wind spinning so violently that a partial vacuum develops within the funnel. A tornado has the most extreme pressure gradients known (as much as a 100-millibar difference between the tornado center and the air immediately outside the funnel). The extreme pressure difference produces winds of extraordinary speed. No one knows how fast the winds in a tornado are because tornadoes blow to bits anemometers (instrument for measuring speed). Maximum estimates range from 320 to 800 kilometers (200 to 500 miles) per hour.

31. What is a funnel cloud?

The rotating vortex of a tornado becomes visible when upswept water vapor condenses. This phenomena is referred to as a funnel cloud.

32. Discuss the general formation of a tornado from a supercell thunderstorm and mesocyclone.

The exact mechanism of formation is unknown. They usually develop in the warm, moist, unstable air associated with a midlatitude cyclone. High wind shear (horizontally rotating air) may cause strong updrafts to form in a supercell thunderstorm. The rotating air may then be tilted vertically, forming a mesocyclone. About 50 percent of all mesocyclones result in tornadoes. These most often develop along a squall line that preceded a rapidly advancing cold front, or along the cold front. Spring and early summer are favorable for development because there’s considerable air-mass contrast present in the midlatitudes at that time. Likewise, most occur in midafternoon, at the time of maximum heating. There is also a region bias in that more than 90 percent of all reported tornadoes occur in the United States. The regionality of this phenomenon is a result of optimum environmental conditions: The relatively flat terrain of central and southeastern United States provides uninhibited interaction of Canadian cP and Gulf mT air masses.

33. Briefly explain the Enhanced Fujita (EF) Scale for tornadoes.

This scale is used to describe the strength of a tornado. The EF scale is based on estimates of 3second gust wind speeds as determined by observed damage after a tornado. See Table 7-4 for the full characteristics of the scale.

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34. What is the difference between a tornado and a waterspout?

True tornados form over land, whereas waterspouts form over water. Waterspouts likewise have a lesser pressure gradient, gentler winds, and a reduced destructive capability compared with tornados.

35. What is the difference between a storm watch and a storm warning?

Storm watch is an advisory issued for a region where over the next 4 to 6 hours the conditions are favorable for the development of severe weather.

Storm warning is issued by a local weather forecasting office when a severe thunderstorm or tornado has actually been observed.

Study Questions 1. Why is an air mass unlikely to form over the Rocky Mountains of North America?

Mountainous areas, areas that vary in altitude and/or covering, and areas where air above moves or is cyclonic are least likely to produce air masses. To develop, air masses need an extensive uniform land or sea surface that allows air to stagnate and develop homogeneous physical characteristics. Ideal source regions are ocean surfaces and extensive flat land areas that have a uniform covering of snow, forest, or desert.

2. Why are maritime polar (mP) air masses from the Atlantic Ocean less important to the United States than mP air masses from the Pacific Ocean?

Air masses that develop over the North Atlantic rarely affect North America because the prevailing circulation is westerly, that is, blowing east (away from the United States). The exception is occasional incursions into the mid-Atlantic coast region. By contrast, mP air masses from the Pacific Ocean affect U.S. weather, with their impact depending on the season. They normally bring widespread cloudiness and heavy precipitation to the mountainous coastal regions; by the time they reach the continent’s interior, they provide moderate temperatures and clear skies. In summer, they produce fog and low stratus clouds along the coast but take no distinctive weather conditions to the interior.

3. Explain why clouds develop along cold fronts and warm fronts.

Both cold fronts and warm fronts serve as atmospheric lifting mechanisms. As these fronts advance, air is uplifted, and it cools adiabatically and eventually condenses. See question 5 for the process.

4. Why do midlatitude cyclones develop in the midlatitudes but not in the tropics?

Midlatitude cyclones occur in equal numbers in both the Northern and Southern Hemispheres, throughout the zone of the westerlies. Their occurrence and distribution are affected by the 196 © 2017 Pearson Education, Inc.

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seasons, so they are more numerous, better developed, and faster moving in each hemisphere’s winter. Because the tropics are a latitudinal region possessing no seasonality, midlatitude cyclones do not develop there.

5. Why are there no fronts in a midlatitude anticyclone?

Fronts are the result of unlike air masses meeting. In a midlatitude anticyclone, no such air-mass conflicts or convergences are involved. Instead, a midlatitude anticyclone is like any other highpressure center, in which air converges into it from above, subsides, and then diverges at the surface. Fronts in the region of a midlatitude anticyclone are technically outside the system.

6. Why are there no fronts in a hurricane?

There are no fronts in a tropical cyclone because it is fairly homogenous in temperature. Being made up of warm, moist tropical air that is condensed into a tight spiral, there is little temperature contrast within it for it to possess distinct fronts.

7. Why are tropical cyclones common along the east coasts of continents in the midlatitudes but not along the west coasts?

Hurricanes sometimes survive (with diminished intensity) off the east coast of continents in the midlatitudes because the warm ocean currents there contribute energy to the cyclones; hurricanes do not survive in the midlatitudes off the west coasts of continents because of the cool ocean currents there.

8. Why is it very unlikely that a hurricane could cross the equator?

Hurricanes follow the path of the trade winds, which do not cross the equator, then bend poleward influenced by westerlies.

9. Why are thunderstorms more common over land than over water?

Thunderstorms occur much more frequently over land than water because summer temperatures are higher over land; most thunderstorms occur in the summer.

10. Why are thunderstorms sometimes called “convective storms”?

Most thunderstorms are caused by localized convective heating; therefore, they are sometimes referred to as convective storms.

Exercises 1. If you see a flash of lightning and you hear thunder 20 seconds later, how far away are you from the lightning in the thunderstorm? _____ miles 5-second delay = 1 mile, so 20 seconds would equal approximately 4 miles.

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2. You see a flash of lightning, and you hear thunder 20 seconds later. Four minutes after the first flash of lightning, you see another flash from the same storm, but the thunder arrives in only 15 seconds. How fast is the thunderstorm moving toward you? _____ miles per hour 20-second delay = 4 miles distant 15-second delay = 3 miles distant 1 mile in 4 minutes = 15 miles per hour 3. Look at the map of thunderstorm activity in the United States (Figure 7-28). Explain why the west coast of California has so little thunderstorm activity while Florida has so much.

Thunderstorms are caused by moist unstable air. In Florida and along the Gulf Coast, moist unstable air prevails in the spring and summer; whereas along the Pacific coast, cool water and subsidence from the subtropical high lead to stable conditions.

Answers to Seeing Geographically Questions (p. 174) Questions: This springtime tornado near Campo, Colorado, produced winds of 210 kilometers per hour (130 miles per hour). Describe the general topography of this region. Do the storm clouds appear to be uniformly thick everywhere in the sky? How does the appearance of the tornado vary from the base of the clouds to the ground?

Answers: The area in which this tornado formed is flat with low-lying vegetation minimizing friction. The tornado funnel narrows near the ground surface. Closer to the ground surface, debris and dust fill the area surrounding the funnel, with the greatest concentration of debris nearest the ground surface. The funnel appears darker toward the bottom, indicating higher density.

Answers to Seeing Geographically Questions (p. 203) Questions: Look again at the photograph of the tornado at the beginning of the chapter (p. 174). How might the topography of this region influence the likelihood of the tornadoes? Why are the spring and early summer the most common times for tornadoes? Why does the funnel cloud look different near the cloud base than where it comes in contact with the ground?

Answers: Tornado outbreaks are much more likely to occur in a flat humid area such as this portion of Colorado. The topography of the Great Plains has few barriers that preclude the intrusion of the warm moist mT air into the plains, nor does the flat topography prevent southern intrusions of cold, dry cP air. The absence of topographic barriers allows both air masses to intrude and meet “head on” in the Great Plains. Late spring and early summer are the most common times of year

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for their formation because during this time of year there is the greatest contrast between cold, dry cP air to the north and moist, warm mT air clashing with it from the south.

Suggested Resources: - Tornadoes: http://www.nssl.noaa.gov/education/svrwx101/tornadoes/ - Hail: http://www.nssl.noaa.gov/research/hail/ https://www.youtube.com/watch?v=xG_orGqMKV8 - Hurricanes: http://www.nhc.noaa.gov/ The following media are available for this chapter in MasteringGeography for student self study and for teachers to assign with assessments: Geoscience Animations: - Cold Fronts - Warm Fronts - Midlatitude Cyclones - Hurricanes - Hurricane Hot Towers - Tornadoes Videos: - 2005 Hurricane Season - Hurricane Sandy

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