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Ch. 3: Weather Patterns Sect. 1: Air Mass & Fronts Sect. 2: Storms Sect. 3: Predicting the Weather Sect. 4: Weather forecasters use advanced technologies


Ch. 3 Weather Fronts and Storms Objective(s) • 7.E.1.3 - Explain the relationship between the movements of air masses; high and low pressure systems, and frontal boundaries to storms (including thunderstorms, hurricanes, and tornadoes) and other weather conditions that may result.

• 7.E.1.4 - Predict weather conditions and patterns based on information obtained from: – • Weather data collected from direct observations and measurement (wind speed and direction, air temperature, humidity and air pressure) – • Weather maps, satellites and radar – • Cloud shapes and types and associated elevation


Learners Objective(s) • “You will understand that…storms are the result of interactions between moving air masses, high and low pressure systems and frontal boundaries.“ • • • • •

Essential Vocabulary -Air mass -Front -Pressure systems -Weather patterns *remaining vocabulary terms located at mrspruillscience.weebly.com


I. Sect. 3.1: Weather Changes as air masses move • A. Air masses are large bodies of air • Air mass – is a large volume of air in which temperature, humidity and air pressure are nearly the same in different locations at the same altitude • An air mass forms when the air over a large region of Earth sits in one place for many days • Where Earth’s surface is cold, the air becomes cold • Where Earth’s surface is wet, the air becomes moist • As an air mass moves, it brings its temperature and moisture to new locations


B. Characteristics of an Air mass – Air masses are classified by 2 characteristics 1. Humidity 2. Temperature

• A hot desert produces dry hot air masses, while cool ocean waters produce moist, cool air masses • Each category name is made of two words – one for moisture, one for temperature • First word tells whether the air mass formed over water or dry land. The 2nd word tells whether an air mass formed close to the equator (temperature)


– The characteristics of an air mass depend on the temperatures and moisture content of the region over which the air mass formed. o Continental: air masses formed over land  Air becomes dry as it loses its moisture to the dry land below it

o Maritime: air masses formed on oceans or seas  Air becomes moist as it gains water vapor from the water below it

o Tropical: warm, air masses formed in the tropics  Air becomes warm as it gains energy from the warm land or water

o Polar: cold, air masses formed north or south of 50º latitude  Air becomes cool as it loses energy to the cold land or water


The colder the air the higher the air pressure subsequently the hotter the air the lower the air pressure. • Cold air  more dense • Hot air  less dense

Types of Air masses –

There are 4 major types of air masses that affect the weather of the U.S.

o Maritime tropical - air mass that is moist and warm o Continental polar - air mass that is dry and cold o Maritime polar - air mass that is moist and cold o Continental tropical - air mass that is dry and warm


C. Movement of an Air Mass

- 2 primary methods for air mass movement 1. Prevailing Westerlies – Pushes air masses from west to east.

1. Jet streams – Pushes fast moving air masses from west to east.


• When air masses move to a new region, it carries along its characteristic moisture and temperature • As the air moves over Earth’s surface, the characteristics of the surface begin to change the air mass

http://www.youtube.com/watch?v=vPC5i6w3yDI&feature=related


D. Weather changes where air masses meet • When a new air mass moves over your area, you can expect the weather to change •

1. 2. 3.

Fronts are the boundary between two air masses. Storms & different types of weather phenomena occur along fronts. – Air masses do not easily mix with each other due to the differences in… Density (Air pressure) Temperature Moisture content • Weather near a front can differ from the weather inside the rest of an air mass • Clouds can form in this rising air


• Types of Fronts •

Cold front - a cold dense air mass that pushes warmer air upward –

Occurs when a fast moving cold air mass overtakes a slower moving warm air mass. o Can move into regions quickly o Often produce tall cumulonimbus clouds and precipitation o Brief, heavy storms are likely and after it, the air is cooler and often very clear


2. Warm front - warm air masses that push colder air upward – Occurs when a fast moving warm air mass overtakes a slow moving cold air mass. o Produces cloud-covered skies – high cirrus and stratus and low stratus clouds o Often brings many hours of steady rain or snow and as it passes the air is warmer


3. Stationary front - occur when air masses first meet or when a cold or warm front stops moving – Occur when a cold and warm air mass meet but neither can move the other. – A wide variety of weather can be found along a stationary front including clouds, prolonged precipitation, fog, and storms. o Produces clouds that cover the sky, sometimes for days at a time


4. Occluded front – Occurs when a warm air mass is caught between 2 cooler air masses. – A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass.


E. High-Pressure Systems • Letter H represents high-pressure systems or highs • Letter L represents low-pressure systems or lows • Each center is the location of the highest or lowest pressure in a region • At a high-pressure center, air sinks slowly down. As the air nears the ground, it spreads out toward areas of lower pressure • High pressure system – is formed when air moves all the way around a high-pressure system o Are often large and change slowly o When one stays in one location for long time, air mass may form – warm or cold, moist or dry o Often brings clear skies and calm air or gentle breezes


F. Low-Pressure Systems • Low-pressure system – is a large weather system that surrounds a center of low pressure o It begins as air moves around and inward toward the lowest pressure and then up to higher altitudes o Rising air produces stormy weather o In the northern hemisphere, the air in a low-pressure system circles in a counterclockwise direction

Pressure System Weather http://www.youtube.com/watch?v=3HAth0D0Pt4&feature=related


II. Section 3.2 – Low-pressure systems can become storms A. Hurricanes form over warm ocean water • Near the equator, warm ocean water provides the energy that can turn a low-pressure center into a violent storm • Tropical storm – is a low-pressure system that starts near the equator and has winds that blow at 40mph or more • Hurricane – is a tropical low-pressure system with winds blowing at speeds of 74 mph or more o Are called typhoons or cyclones when they form over the Indian Ocean or the western Pacific Ocean


B. Formation of Hurricanes • Energy from warm water is necessary for a lowpressure center to build into a tropical storm and then into a hurricane • Tropical storms generally move westward with the trade winds • As long as the storm stays over warm water it can grow bigger and more powerful • Once the hurricane moves over land or cooler water, it loses its source of energy • Eye – at the center of a hurricane that is a small area of clear weather (about 20-50 kilometers/1030 miles in diameter) Anatomy of A Hurricane http://www.youtube.com/watch?v=HJydFJORWf4&feature=related


• C. Effects of Hurricanes • In the space below, list several effects that a hurricane can have – – – – –

Lift cars Uproot trees Tear the roofs off buildings May produce tornadoes Can cause river banks to overflow and flood nearby areas

• Storm Surge http://www.youtube.com/watch?v=gd8WiiXNxho&feature=relmfu – Huge mass of ocean water. – Sea levels rise several meters, backing up rivers and flooding the shore – Can be destructive and deadly Above the storm http://www.youtube.com/watch? v=eT5K6FR_eVs&feature=related

How hurricanes are formed!!! http://www.youtube.com/watch?v=4f45jA5UxB0


Stages of Hurricane Development 1. 2. 3. 4.

Stage 1: Tropical disturbance; 10-23 mph Stage 2: Tropical depression; 23-39 mph Stage 3: Tropical storm; 40-73 mph Stage 4: Hurricane; 74 mph

The Saffir-Simpson scale –

Scale used to determine the severity of a hurricane. • • • • •

Category 1: wind speed 74-95 mph; storm surge 4-5 feet. Category 2: wind speed 96-110 mph; storm surge 6-8 feet. Category 3: wind speed 111-130 mph; storm surge 9-12 feet. Category 4: wind speed 131-155 mph; storm surge 13-18 feet. Category 5: wind speed 155+ mph; storm surge 18+ feet –

Hurricane Katrina was a category 3 hurricane when it made landfall near New Orleans, La on August 29, 2008. » Costliest natural disaster » 6th strongest to form, 3rd strongest to make landfall » 1 of the 5 deadliest


– Hurricanes • Tropical cyclone (low pressure) that typically measures 300-500 miles across with winds from 70-200 mph. • Comes from the West Indian word Huracan or “big wind.” • Called Typhoons when formed in the Pacific Ocean; – Chinese word, Táifēng or “great wind.”

• Hurricanes are named by the World Meteorological Organization. • Guided or directed by the Trade winds. • Can only form over water that is at least 80ºF. • Typically forms during the months of late July to early October.


D. Winter Storms produce snow and ice • Most severe winter storms in the US are part of lowpressure systems • The systems that cause winter storms are formed when two air masses collide • Blizzards – Are blinding snowstorms with winds of at least 35 mph and low temperatures (usually below 20 degrees F) – Occur in many parts of the northern and central US – Wind and snow can knock down trees and power lines – Water pipes can freeze


• Lake-effect Snowstorms – Heavy snow fall in the areas just east and south of the Great Lakes – Cold air from the NW gains moisture and warmth as it passes over the Great Lakes – Over cold land, the air cools again and releases the moisture as snow http://www.youtube.com/watch?v=pQeWSl3laXI


• Ice Storms o Cold rain freezes as it touches the ground and other surfaces with a heavy, smooth ice o Ice-covered roads become slippery and dangerous o Drivers may find it hard to steer and to stop their cars o Branches or even whole trees may break from the weight of ice o Falling branches can block roads, tear down power and telephone lines, etc o Damage from ice storms can sometimes shut down entire cities


III. Section 3.3 – Vertical air motion can cause severe storms http://www.youtube.com/watch? v=NxWbR60tFlg&feature=relmfu

A. Thunderstorms form from rising moist air • Electrical charges build up near the tops and bottoms of clouds as pellets of ice move up and down through the clouds • A charge sparks from one part of a cloud to another or between a cloud and the ground • Lightning – a spark of electricity and causes a bright flash of light • Air around lightning is briefly heated to a temperature hotter than the surface of the Sun • This fast heating produces a sharp wave of air that travels away from the lightning • When the wave reaches you, you hear it as a crack of thunder • Thunderstorm – is a storm with lightning and thunder


B. Formation of Thunderstorms http://www.youtube.com/watch?v=exlVSEPEXKc

• Rising humid air forms a cumulus cloud. The water vapor release energy when it condenses into cloud droplets. This energy increases the air motion. The cloud continues building up into the tall cumulonimbus of a thunderstorm • Ice particles from in the low temperatures near the top of the cloud. As the ice particles grow large, they being to fall and pull cold air down with them. This strong downdraft brings heavy rain or hail – the most severe stage of a thunderstorm • The downdraft can spread out and block more warm air from moving upward into the cloud. The storm slows down and ends. • Can form at a cold front or within an air mass • Conditions that produce thunderstorms occur almost daily during part of the year


C. Effects of Thunderstorms • Flash Floods http://www.youtube.com/watch?v=cxAUoXTUtS8 o Can be strong enough to wash away people, cars, and even houses http://www.youtube.com/watch?v=vfT5dEKaUdk o One thunderstorm can produce millions of liters of rain o Water can cover the ground or make rivers

• Winds o Can be very strong during a thunderstorm o Can blow in bursts and exceed 270km/hour (170mph)

• Hail

http://www.youtube.com/watch?v=xG_orGqMKV8

o Causes nearly $1 billion in damage to property and crops in the US every year o Can wipe out entire fields of a valuable crop in a few minutes o Can damage roofs and kill livestock

• Lightning o Can kill or seriously injure any person it hits o Can spark dangerous forest fires


– Thunderstorms –Lightning: sudden spark or electrical discharge

typically caused by the build up of positive charges on Earth with negative charges within the air.

» Cloud to cloud » Cloud to ground » Ground to cloud (rare) – Thunder is caused as air is superheated (30,000ºC), expands, and explodes. » Thunder is the sound wave created from the explosion. » Because sound travels slower than light, thunder always comes after lightning not the other way around.


D. Tornadoes form in severe thunderstorms • Tornado – is a violently rotating column of air stretching from a cloud to the ground • Moves along the ground in a winding path underneath the cloud • The column may even rise off the ground and then come down in a different place • May become visible when water droplets appear below the cloud in the center of a rotating column • May lift dust and debris from the ground, so the bottom of the column becomes visible • May form a series of tornadoes or even a group of tornadoes all at once • • •

How tornadoes are formed!!! http://www.youtube.com/watch?v=MjjwPQRVbWk&feature=relmfu http://www.youtube.com/watch?v=d9MyV3fGP4M&feature=related http://www.youtube.com/watch?v=006guBgSf14&feature=related


– Tornadoes • Tornadoes can form in any situation that produces severe weather. • Typically form during the Spring & Summer under the same conditions as those of a thunderstorm. • Tornado formation – Warm, moist air flows in at the bottom of a cumulonimbus cloud & rapidly moves upward generating a low pressure area inside the cloud. – The warm air begins to rotate due to winds within the cloud blowing in different directions: The result is the cloud begins to spin like a top. – As part of the cloud descends to touch the ground, a tornado or funnel cloud is generated with winds up to 340 mph. – http://www.youtube.com/watch?v=d9MyV3fGP4M&feature=related


• The Fujita Scale – Used to determine the severity of a tornado. – Based on the amount of damage created as well as the wind speed. • • • • • •

F-0: Gale tornado, 40-72 mph winds F-1: Moderate tornado, 73-112 mph winds F-2: Significant tornado, 113-157 mph winds F-3: Severe tornado, 158-206 mph winds F-4: Devastating tornado, 207-260 mph winds F-5: Incredible tornado, 261-300+ mph winds

• Tornado alley – Located in the Midwest region of the U.S. & is known for the development of tornadoes. – Includes the states of S. Dakota, Iowa, Nebraska, Kansas, Oklahoma, and Texas.


E. Effects of Tornadoes • Can cause damage as the bottom of the tornado moves along the ground • Can also pick up and slam dirt and small objects into buildings or anything else in the tornado’s path • Most common tornadoes are small and last only a few minutes • Larger ones are less common but have stronger winds and last longer • 20% are strong enough to knock over large trees, lift cars off the ground, and tear the roof off houses • Moves along with its thunderstorm • National Weather Service issues a tornado watch when the weather condition might produce tornadoes. A warning is issued when a tornado has been detected


– Winter storms • Lake effect snow – Caused as cold dry air moves across a warmer body of water. – It becomes more humid as water vapor evaporates from the lake surface. – The air reaches land & cools causing lake-effect snow to fall. » Great Lakes area (Michigan, Wisconsin, & Buffalo, NY) Lake Effect Snow

http://www.youtube.com/watch?v=Pp-3kq-5EvU


IV. Section 3.4 – Weather forecasters use advanced technologies A. Weather data come from many sources • Weather report can show conditions in your area and also in the region around you • Meteorologist – is a scientist who studies weather • Looks at past and current conditions. They use many forms of technology to gather data • Types of technology that is used to collect weather data o Radar stations – locate clouds and measure their heights. Doppler radar can detect air motion and precipitation o Airplanes and ships – can carry instrument packages that make measurements wherever they go


o Ground stations – hold instruments that measure air pressure, temperature, dew point, precipitation, wind speed, wind direction, and cloud cover o Satellites – orbit Earth above the atmosphere. Images can show cloud cover, warm and cool regions, and invisible water vapor o Weather balloons – make important measurements of the air at different altitudes as they carry instruments high into the stratosphere o Weather buoys – record the weather far from cities. They also measure conditions in the ocean that affect the atmosphere • TIROS-1: 1st weather satellite launched in 1960 • Located within the exosphere • Carries a variety of scientific weather instruments – Cameras: to take pictures of various weather factors. – Temperature gauges, Radiation detectors, humidity detectors, wind speed gauges, etc.


B. Weather data can be displayed on maps • Scientists use computers to record and use the enormous amount of data gathered • Meteorologists use maps to display a lot of information at once

C. Air pressure on weather maps • Isobar – is a line that connects places that have the same air pressure • Each isobar represents a different air pressure value http://www.youtube.com/watch?v=hKGQJEp-Pys


• Each is labeled with the air pressure for that whole line in units called millibars • Shows pressure patterns which determine winds

• Lines close together show a big difference in air pressure. Expect strong winds here. • Lines are far apart where the air pressure is almost even. Expect calm air or light breezes near the high – Information shown on a weather map • • • • • •

Precipitation type Fronts Temperature Air pressure Cloud cover High pressure and low pressure area


D. Forecasters use computer models to predict weather • Instruments can only measure the current weather conditions • Forecasters can make some predictions from their own observations • If they have information from other places, the forecasters might be able to tell where the warm front is already and how fast it is moving • They might be able to predict how soon it will arrive and even how warm the weather will be after the front passes http://www.youtube.com/watch?v=dqpFU5SRPgY&feature=related


• Computers have become an important tool for forecasting weather • When weather stations send in data computers can create maps right away • They combine many types of date to forecast what might happen next • Scientists study the computer forecasts, then apply their knowledge and experience to make weather predictions • Forecasting the weather is complicated • The farther in advance a forecast is made, the more time there is for small differences between the predicted and the actual weather to add up • Up to 3 days in advance are the most accurate


Weather Fronts and Storms  

Chapter 3 on Weather

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