Air Masses and Fronts (pages 130–137)

Types of Air Masses (pages 131–132)

Key Concept: Four major types of air masses influence the weather in North America: maritime tropical, continental tropical, maritime polar, and continental polar.

• An air mass is a huge body of air. The air at any given height in an air mass all has about the same temperature, humidity, and air pressure.

• A tropical air mass forms in the tropics. A tropical air mass has warm air. A polar air mass forms near the poles. A polar air mass has cold air.

* continued

• A maritime air mass forms over an ocean. A maritime air mass has humid air. A continental air mass forms over land. A continental air mass has dry air.

• A maritime tropical air mass has warm, humid air. A maritime polar air mass has cool, humid air.

• A continental tropical air mass has hot, dry air. A continental polar air mass has cold, dry air.

Maritime-related to the sea

Continental-land

Weather Patterns - Air Masses and Fronts

Classifying Air Masses

Four major types of air

masses influence the

weather in North America:

maritime tropical, continental

tropical, maritime polar, and

continental polar.

Weather Patterns - Air Masses and Fronts

Types of Air Masses

Air masses can be warm or cold, and humid or dry. As an air mass moves into an

area, the weather changes.

How Air Masses Move (page 133)

Key Concept: In the continental United States, air masses are commonly moved by the prevailing westerlies and jet streams.

• Air masses move. Global winds usually move air masses from one part of Earth to another.

• The major wind belts over the United States are the prevailing westerlies. These winds usually push air masses from west to east across the United States.

• Jet streams also push air masses across the United States from west to east.

• As air masses move, they bump into each other. A front is a boundary where two air masses meet. Storms and other types of weather develop along fronts.

Weather Patterns - Air Masses and Fronts

How a Front Forms

The boundary where unlike air masses meet is called a front.

Types of Fronts (pages 134–135)

Key Concept: Colliding air masses can form four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.

• A cold front forms when a cold air mass slides under a warm air mass. Thunderstorms can occur at a cold front.

• A warm front forms when a warm air mass moves over a cold air mass. Light rain or snow may fall.

• Sometimes cold and warm air masses meet, but neither air mass moves over or under the other. The two air masses face each other in a “standoff.” This is called a stationary front. A stationary front can bring many days of clouds and precipitation.

*continued

• A warm air mass sometimes is caught between two cooler air masses. This forms an occluded front. Occluded means to be cut off.

In an occluded front, the warm air mass is cut off from the ground. The weather may turn cloudy, and rain or snow may fall.

Weather Patterns - Air Masses and Fronts

Types of Fronts

Colliding air masses can form four types of fronts: cold fronts, warm fronts,

stationary fronts, and occluded fronts.

Weather Patterns

Comparing and Contrasting

As you read, compare and contrast the four types of fronts

by completing a table like the one below.

Cold front A cold air mass

overtakes a warm air

mass.

Front How It FormsTypes of

Weather

Clouds, possibly storms

with heavy precipitation

Warm front A warm air mass

overtakes a cold air

mass.

Clouds, light

precipitation

Stationary front Cold and warm air

masses meet, but neither

can move the other.

Clouds, precipitation

Occluded front A warm air mass is caught

between two cold air

masses.

Clouds, precipitation

- Air Masses and Fronts

Cyclones and Anticyclones (pages 136–137)

Key Concept: Cyclones and decreasing air pressure are associated with clouds, winds, and precipitation.

• A cyclone is a swirling center of low air pressure. The winds in a cyclone spiral around a center. In a cyclone, winds spiral inward towards the low-pressure center.

• The Coriolis effect makes winds spin counterclockwise in a Northern Hemisphere cyclone.

• Air spins upward in a cyclone. The result is clouds, wind, and precipitation.

• Cyclones play a large part in the weather of the United States.

Key Concept: The descending air in an anticyclone generally causes dry, clear weather.

• An anticyclone is a high-pressure center of dry air. An anticyclone is the opposite of a cyclone. In an anticyclone, winds spiral outward away from the high-pressure center.

• The Coriolis effect makes winds spin clockwise in a Northern Hemisphere anticyclone.

• The air spins down in an anticyclone. The result is dry, clear weather.

Weather Patterns - Air Masses and Fronts

Cyclones and Anticyclones

Winds spiral inward towards the low-pressure center of a

cyclone. Winds spiral outward from the high-pressure center

of an anticyclone.

Storms (pages 138–147)

Thunderstorms (pages 139–140)

Key Concept: Thunderstorms form in large cumulonimbus clouds, also known as thunderheads.

• A storm is a violent disturbance in the atmosphere. Storms include thunderstorms, tornadoes, hurricanes, and winter storms.

• A thunderstorm is a small storm that often has heavy precipitation and thunder and lightning.

• Cumulonimbus clouds form on hot, humid afternoons. They also form along cold fronts.

• Lightning is a sudden spark in the atmosphere. Lightning can cause an explosion of air. Thunder is the sound of that explosion.

Weather Patterns - Storms

Thunderstorm Formation

A thunderstorm forms when warm, humid air rises rapidly within a cumulonimbus

cloud.

Key Concept: During thunderstorms, avoid places where lightning may strike. Also avoid objects that can conduct electricity, such as metal objects and bodies of water.

• The safest place to be during a thunderstorm is indoors.

• Metal objects can conduct electricity. Water also can conduct electricity. Stay away from objects that can conduct electricity during thunderstorms.

Tornadoes (pages 140–143)

Key Concept: Tornadoes most commonly develop in thick cumulonimbus clouds—the same clouds that bring thunderstorms.

• A tornado is a rapidly spinning cloud that reaches down to Earth’s surface.

• Tornados usually form in cumulonimbus clouds. A tornado cloud is shaped like a funnel—wide at the top and narrow at the bottom.

• Tornadoes occur more often in the United States than in any other country. They are common on the Great Plains. “Tornado alley” includes parts of Texas, Oklahoma, Kansas, Nebraska, and South Dakota.

Weather Patterns - Storms

Tornado Alley

Tornadoes in the U.S. are most likely to occur in a region known as Tornado Alley.

Key Concept: The safest place to be during a tornado is in a storm shelter or the basement of a well-built building.

• A “tornado watch” is an announcement that tornadoes are possible in your area. A “tornado warning” is an announcement that a tornado has been seen in your area.

• If a building does not have a basement, move to the middle of the lowest floor during a tornado.

Hurricanes (pages 144–145)

Key Concept: A hurricane begins over warm ocean water as a low-pressure area, or tropical disturbance.

• A hurricane is a tropical cyclone. A cyclone has low pressure. A hurricane has winds that are at least 119 kilometers per hour. Hurricanes form in the Atlantic, Pacific, and Indian oceans.

• A hurricane draws its energy from the warm, humid air at the ocean’s surface. The center of a hurricane is called the “eye.” The eye is calm.

• Hurricanes last a week or more. Hurricanes that form in the Atlantic Ocean are pushed by easterly trade winds toward the southeastern United States.

Weather Patterns - Storms

Structure of a Hurricane

In a hurricane, air moves rapidly around a low-pressure area called the eye.

Weather Patterns

Sequencing

As you read, make a flowchart like the one below that shows how a hurricane forms. Write each step of the process in a separate box in the order in which it occurs.

Hurricane Formation

Begins as a low-pressure area over warm water,

or a tropical disturbance.

Warm, humid air rises and begins to spiral.

As air rises, more warm, moist air is drawn into the

system and the hurricane gains energy.

As winds spiral inward, bands of high winds

and heavy rains form.

- Storms

Weather Patterns

PDI, or power dissipation index

The graph shows the energy

released annually by all storms

in the North Atlantic compared

to the September average sea

surface temperature (SST) over

a region known to generate

storms.

The correlation between the lines suggests that sea

surface temperature influences how much energy storms

release.

Key Concept: If you hear a hurricane warning and are told to evacuate, leave the area immediately.

• To evacuate means to move away from an area.

• If you have to stay in a house, move away from the windows.

Winter Storms (pages 146–147)

Key Concept: All year round, most precipitation begins in clouds as snow. If the air is colder than 0°C all the way to the ground, the precipitation falls as snow.

• In warmer months, the snow that falls from clouds melts and then hits Earth’s surface as rain.

• Cities near the Great Lakes can receive a lot of snow

Key Concept: If you are caught in a snowstorm, try to find shelter from the wind.

• Blowing snow can limit how much you can see.

• The winds in a winter storm can cool your body.

Weather Patterns - Storms

Lake-Effect Snow

As cold dry air moves across the warmer water, it becomes more humid as water

vapor evaporates from the lake surface. When the air reaches land and cools,

snow falls.

Review

• Low Pressure = Stormy Weather

• As the air rises, the water vapor within it condenses forming clouds and often precipitation too.

• A high pressure system has higher pressure at its center than the areas around it. Wind blows away from high pressure. Winds of a high pressure system swirl in the opposite direction as a low pressure system - clockwise north of the equator and counterclockwise south of the equator.

Cloud review

• Nimbus-Low hanging rain cloud

• Stratus-Meaning Layer

• Cumulus-Heap or pile

• Cirrus-Hair-can also be an indication of a change in weather patterns within the next 24 hours (most likely a change of pressure fronts)

Predicting the Weather (pages 150–156)

Weather Forecasting (page 151)

Key Concept: Meteorologists use maps, charts, and computers to analyze weather data and to prepare weather forecasts.

• A meteorologist (mee tee uh RAHL uh jist) is a scientist who studies the causes of weather and predicts the weather. Meteorologists analyze weather data, which means that they organize weather information and try to see trends.

• Most weather information comes from the National Weather Service, a part of the U.S. government. The National Weather Service uses scientific instruments to gather weather data.

Weather Technology (pages 152–153)

Key Concept: Technological improvements in gathering weather data and using computers have improved the accuracy of weather forecasts.

• Weather balloons carry scientific instruments up into the atmosphere. The instruments measure temperature, air pressure, and humidity.

• Weather satellites orbit Earth in the upper layer of the atmosphere, called the exosphere.

• The National Weather Service has built many weather observation stations. These stations collect data about temperature, humidity, rainfall, and winds.

• Computers are used to process weather data and help weather forecasters make predictions.

Review Tools

• Thermometer

• Psychrometer

• Rain gauge

• Barometer

• Anemometer

• Wind vane

Weather Patterns - Predicting the Weather

Red Sky

A red sky is one kind of observation that helps people to predict the weather.

Weather Patterns

Computer Weather Forecasting

Scientists use computers to develop different models of how a front may move. These predictions are then used to make weather forecasts. As more data become available, some models are found to be incorrect, while others are found to closely fit the predicted conditions. The upper graph shows the predicted air pressure from two models. The lower graph shows actual data for air pressure.

- Predicting the Weather

Weather Patterns

Computer Weather Forecasting

Time of day and air pressure

Reading Graphs:

What two variables are being

graphed?

- Predicting the Weather

Weather Patterns

Computer Weather Forecasting

Model B

Inferring:

Which computer model most

closely matches the actual air

pressure data?

- Predicting the Weather

Weather Patterns

Computer Weather Forecasting

Stormy weather, clouds, and precipitation accompany low air pressure.

Predicting:

What weather would you

forecast for Monday and

Tuesday? Explain.

- Predicting the Weather

Reading Weather Maps (pages 154–156)

Key Concept: Standard symbols on weather maps show fronts, areas of high and low pressure, types of precipitation, and temperatures.

• A weather map is a “snapshot” of weather conditions at a particular time over a large area.

• The National Weather Service collects data from weather stations all over the country. This data can be shown on a weather map.

• Thin lines on a weather map connect places with the same air pressure or temperature. An isobar is a line on a weather map that connects places with the same air pressure. An isotherm is a line on a weather map that connects places with the same temperature.

Weather Patterns - Predicting the Weather

Reading Weather Map Symbols

The figure below shows what various weather symbols mean.

Weather Patterns - Predicting the Weather

Reading Weather Maps

This is the type of weather map produced by the National Weather Service. It

shows data collected from many weather stations.

Weather Patterns - Predicting the Weather

Reading Weather Maps

Weather maps in newspapers use symbols to show fronts, high- and low-pressure

areas, and precipitation. Color bands indicate different temperature ranges.