Miss Nelson

SCIENCE ~ CHAPTER 8

WEATHER

Air Masses and Fronts

SECTION 3

Which is denser… warm air, or cold air?

What do you think happens when warm air and cold air collide?

ANTICIPATORY SET

S 6.4.e –Students know differences in pressure, heat, air movement, and humidity result in change in weather

STANDARDS

What are the major types of air masses in North America, and how do they move?

What are the main types of fronts?

What type of weather is associated with cyclones and anticyclones?

THE BIG IDEA

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

Tropical – a warm air mass that forms in the tropics and has low air pressure

Polar – a cold air mass that forms north of 50 degrees latitude or south of 50 degrees latitude and has high air pressure

Maritime – a humid air mass that forms over oceans

Continental – a dry air mass that forms over land

KEY TERMS

Front – the boundary where unlike air masses meet but do not mix

Occluded – cut off, as in a front where warm air mass is caught between two cooler air masses

Cyclone – a swirling center of low air pressure

Anticyclone – a high-pressure center of dry air

KEY TERMS

Weather can be influenced by air from thousands of kilometers away

A huge body of air that has similar temperature, humidity, and air pressure at any given height is called an air mass

A single air mass may spread out over millions of square kilometers and be up to 10 kilometers deep

AIR MASSES AND FRONTS

Four major types of air masses influence the weather in North America:Maritime tropicalContinental tropicalMaritime polarContinental polar

TYPES OF AIR MASSES

The characteristics of an air mass depend on the temperatures and moisture content of the region over which the air mass forms

TYPES OF AIR MASSES

Temperatures affects pressure

Cold, dense air has a higher pressure

Warm, less dense air has a lower pressure

TYPES OF AIR MASSES

Tropical air masses:

Warm air masses

Form in the tropics

Have low air pressure

Polar air masses:

Cold air masses

Form north of 50° and south of 50°

Have high air pressure

TYPES OF AIR MASSES

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

TYPES OF AIR MASSES

Maritime air masses form over waterThe air can become very humid

Continental air masses form over landHave less exposure to moisture from water – are drier

TYPES OF AIR MASSES

Warm, humid air masses form over tropical oceansGulf of Mexico, Atlantic Ocean

In summer time, they bring hot, humid weatherSummer showers and thunderstorms

In winter, bring heavy rain or snow

MARITIME TROPICAL

Cool, humid air masses form over the icy cold North Pacific and North Atlantic Oceans

Affect the west coast more than the east coast

Even in summer, they bring fog, rain, and cool temperatures to the west coast

MARITIME POLAR

Hot, dry air masses form mostly in summer over dry areas of the Southwest and northern Mexico

Cover a smaller area than other air masses

Occasionally move northeast, bringing hot, dry weather to the southern Great Plains

CONTINENTAL TROPICAL

Form over central and northern Canada and Alaska

Can bring bitterly cold weather with very low humidity

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

In summer, the air mass is milder

Storms may occur when continental polar air masses move south and collide with maritime tropical air masses moving north

CONTINENTAL POLAR

Read Types of Air Masses on pages 311-312 of your textbook

TYPES OF AIR MASSES

In the continental United States, air masses are commonly moved by the prevailing westerlies (winds) and jet streams

HOW AIR MASSES MOVE

The major wind belts over the continental United States

Generally push air masses from west to east

PREVAILING WESTERLIES

Embedded within the prevailing westerlies are jet streams

Bands of high speed winds about 10 kilometers above Earth’s surface

Blow from west to east

JET STREAMS

As huge air masses move across the land and the oceans, they collide with each otherThey do not easily mix

Think about oil and water!Less dense oil floats on top of the denser water

Something similar happens when two air masses with different temperatures and humilities collide

FRONTS

The boundary where these air masses meet is called a front

Storms and changeable weather often develop along fronts

View the picture on page 313 of your textbook

FRONTS

Read How Air Masses Move on page 313 of your textbook

HOW AIR MASSES MOVE

Colliding air masses can form four types of fronts:

Cold frontsWarm frontsStationary frontsOccluded fronts

TYPES OF FRONTS

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

TYPES OF FRONTS

Cold air is dense and tends to sink

Warm air is less dense and tends to rise

When a rapidly moving cold air mass runs into a slowly moving warm air mass… The denser cold air slides under the lighter warm air The warm air is pushed upward along the leading

edge of the colder air

View figure 14 on page 314 of your textbook

COLD FRONT

As the warm air rises, it expands and cools

Warm air can hold move water vapor than cool air

The rising air reaches the dew point (the temperature at which water vapor in the air condenses into droplets of water)

Clouds form

If there is a lot of water vapor in the warm air, heavy rain or snow may fall

If the warm air mass contains only a little water vapor, then the cold front may be accompanied by only cloudy skies

COLD FRONT

Read Cold Fronts on page 314 of your textbook

COLD FRONT

Clouds and precipitation also accompany warm fronts

At a warm front, a fast-moving warm air mass overtakes a slowly moving cold air mass

Cold air is denser than warm air, the warm air moves over the cold air

If the warm air is humid, l ight rain or snow falls along the front

If the warm air is dry, scattered clouds form

Because warm fronts move slowly, the weather may be rainy or cloudy for several days

After a warm front passes through an area, the weather is l ikely to be warm and humid

WARM FRONTS

Read Warm Fronts on page 315 of your textbook

WARM FRONTS

View the diagram on page 315 of your textbook

Read Stationary Fronts on page 315 of your textbook

STATIONARY FRONTS

View the diagram on page 315 of your textbook

Read Occluded Fronts on page 315 of your textbook

OCCLUDED FRONTS

As air masses collide to form fronts, the boundary between the fronts sometimes becomes distorted

Can be caused by surface features such as mountains, strong winds, or jet streams

When this happens the air begins to swirl

Swirling air can cause a low pressure center to form

CYCLONES AND ANTICYCLONES

When you look at a weather map you will see areas marked with an L Stands for “low” pressure

Swirling center of low air pressure is called a cyclone From the Greek word meaning “wheel”

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

As air rises in the cyclone, the air cools, forming clouds and precipitation

CYCLONES

Cyclones and decreasing air pressure are associated with clouds, winds, and precipitation

CYCLONES

The opposite of a cyclone

High pressure centers of dry air

Called “highs” and marked with an H on a weather map

Winds spiral outward from the center of an anticyclone, moving toward areas of low pressure

ANTICYCLONES

The descending air in an anticyclone generally causes dry, clear weather

ANTICYCLONES

What two characteristics are used to classify air masses?

Of the four types of air masses, which are dry, and which are humid?

CHECK FOR UNDERSTANDING

What two characteristics are used to classify air masses?The two characteristics used to classify air masses are temperature and humidity.

Of the four types of air masses, which are dry, and which are humid?Maritime tropical and maritime polar air masses are humid, while continental tropical and continental polar air masses are dry.

CHECK FOR UNDERSTANDING

What type of air mass would form over the northern Atlantic Ocean?

What are the four types of fronts?

GUIDED PRACTICE

What type of air mass would form over the northern Atlantic Ocean?The type of air mass that would form over the northern Atlantic Ocean is a maritime polar air mass.

What are the four types of fronts?The four types of fronts are cold front, warm front, stationary front, and occluded front.

GUIDED PRACTICE

Complete Weather 8-3 Independent Practice

INDEPENDENT PRACTICE