Previewing the Chapter• Read the titles of the chapter and each

section For Sections 1 and 4:

• Read the headings (the words in green)• Scan the illustrations, charts, tables, and

graphsFor Sections 2 and 3:

• Skim Storms and Floods• CREATE YOUR OWN TITLE PAGE AS

YOU DO THIS.

Air Masses and Fronts

Section One

Pages 76-82

What is an air mass?

Air mass = a huge body of air that has similar temperature, humidity, and air pressure throughout

A single air mass may spread over an area of millions of square kilometers and be up to 10 kilometers high!

Scientists classify air masses according to two characteristics:

1. temperature

2. humidity

Temperature

Whether an air mass is warm or cold depends on the temperature of the region over which the air mass forms.

Tropical Air Mass

• Tropical, or warm air masses form in the tropics.

• They have low air pressure.

What are the tropics?• Regions of the Earth that lie roughly in the

middle of the globe. The tropics are between the latitude lines of the Tropic of Cancer and the Tropic of Capricorn. The tropics are warm all year, averaging 77-82 degrees Fahrenheit.

Polar Air Mass• Polar, or cold, air masses form near the

poles.• They have high air pressure.

What are the polar regions? • The ice-covered areas around the far south

and north ends of the globe, the North and South Pole. The continental regions of Arctic and Antarctica settle in these two polar coordinates respectively. The weather is very cold, far below freezing temperatures, all year around.

Humidity

Whether an air mass is humid or dry depends on whether it forms over water or land.

Maritime Air Mass

• Maritime air masses form over oceans.

• Water evaporates from the oceans which causes the air to become very humid.

Continental Air Mass

• Continental air masses form over land, in the middle of continents.

• They are very dry.

North American Air Masses

Four major types of air masses influence the weather in North America:

1. Maritime Tropical (mT)

2. Maritime Polar (mP)

3. Continental Tropical (cT)

4. Continental Polar (cP)

Maritime Tropical

• They form over the ocean.

• They are warm and wet.

• They influence weather in the central and eastern United States and also along the West Coast.

• In the summer they bring hot, humid weather. In winter, a humid air mass can bring heavy rain or snow.

Maritime Polar

• They form over the ocean.

• They are cool and wet.

• Affect the West Coast more than the East Coast.

• In the summer these masses bring fog, rain, and cool temps to the west coast.

Continental Tropical

• They form over land.

• They are hot and dry.

• Form only in summer over dry areas of the Southwest and northern Mexico.

• Cover a smaller area.

• Occasionally they move NE bringing hot, dry weather to the southern Great Plains.

Continental Polar

• They form over land, central and northern Canada and Alaska.

• They bring cool or cold air.

• In winter, they bring clear, cold, dry air to much of North America.

Discover Activity

With your group members, read the steps in the Discover Activity on page 76.

With your group members, make a prediction by illustrating what will happen when the divider is removed (#4).

Now write a hypothesis stating what would happen if a mass of cold air ran into a mass of warm air. With your group members of course.

How Air Masses Move

The prevailing westerlies are the major wind belts in the continental United States. They generally push air masses from the west to the east.

Fronts

Fronts are the area where air masses meet and do not mix.

Why don’t they mix? Think about the red and blue water…

When air masses of different temperatures and densities collide, the collision causes storms and changeable weather.

Types of Fronts

There are 4 types of fronts:

1. Cold fronts

2. Warm fronts

3. Stationary fronts

4. Occluded fronts

The kind of front that develops depends on the characteristics of the air masses and how they are moving.

Cold Fronts

A cold front forms when cold air moves underneath warm air, forcing the warm air to rise.

Cold fronts move quickly, so they can cause abrupt weather changes, including violent thunderstorms.

Warm Fronts

A warm front forms when warm air moves over cold air. The warm air mass is moving faster than the cold air mass.

Clouds, storms, and rain accompany warm fronts.

http://www.youtube.com/watch?v=huKYKykjcm0

Stationary Fronts

When two air masses meet but neither one has enough force to move the other.

Could bring many days of clouds and precipitation.

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

Occluded Fronts When a cold air mass and a cool air mass

come together, the warm air caught between them is forced upward.

The two cooler air masses may mix while the warm air mass is cut off, or occluded, from the ground.

https://www.youtube.com/watch?v=79_s831Ba5U

Cyclones

A swirling center of low air pressure is called a cyclone.

On a weather map they are marked with an L, for low pressure.

Winds spiral inward toward the center of the system.

Cyclones

Cyclones impact weather in the United States.

Cyclones and decreasing air pressure are associated with storms and precipitation.

Anticyclones

High-pressure centers of dry air On a weather map they are marked with an H,

for high pressure. Winds spin clockwise in the Northern

Hemisphere. Cause dry, clear weather.

Words Worth Knowing (ISN #29)

Air mass Tropical Polar Maritime Continental

Front Stationary Occluded Cyclone Anticyclone