Warm Up 3/31/08 1.True or False: More water vapor can exist in warm air than cold air. 2.Explain briefly how wind forms. 3.What are low, sheetlike clouds.

Warm Up 3/31/08

1. True or False: More water vapor can exist in warm air than cold air.

2. Explain briefly how wind forms.

3. What are low, sheetlike clouds called?

Air Masses

Chapter 20, Section 1

Air Masses and Weather Air Mass – immense body of air that is

characterized by similar temperatures and amounts of moisture at any given altitude

Because of its size, it may take several days for an air mass to move over an area (giving that area fairly constant weather)

When an air mass moves out of the region over which it formed, it carries its temperature and moisture conditions with it

As it moves, the characteristics of an air mass change, and so does the weather in the area over which the air mass moves

Influence of a Canadian Air Mass

Concept Check

What is an air mass, and what happens as it moves over an area?

An air mass is an immense body of air characterized by similar temperatures and amounts of moisture at any given altitude. As it moves, the characteristics of an air mass change and the weather in the area over which the air mass moves also changes.

Classifying Air Masses The area over which an air mass gets its characteristic

properties of temperature and moisture is called its source region

Air masses are named according to their source region Polar (P) air masses (cold air) form at high latitudes, while

tropical (T) air masses (warm air) form at low latitudes In addition to their overall temperature, air masses are

classified according to the surface over which they form Continental (c) form over land (dry air) and maritime (m) form

over water (humid air) The four basic types of air masses in North America:

Continental Polar (cP) – cold and dryContinental Tropical (cT) – warm and dry

Maritime Polar (mP) – cold and moistMaritime Tropical (mT) – warm and moist

Air Mass Source Regions

Concept Check

How are air masses classified?Air masses are classified by temperature

(polar or tropical) and the surface (continental or maritime) over which they form.

Weather in North America Much of the weather in North America, especially weather east

of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses

Continental polar air masses are uniformly cold and dry in winter and cool and dry in summer; lake-effect snow is caused when one of these air masses passes over the moisture of the lake and drops the precipitation down on the leeward side of the lake

Maritime tropical air masses are warm, loaded with moisture, and usually unstable; they are the source of much of the precipitation in the eastern two thirds of the U.S.

During the winter, maritime polar (mP) masses that affect weather in North America come from the North Pacific

Continental tropical air masses have the least influence on the weather of North America

Only occasionally do continental tropical (cT) air masses affect the weather outside their source region

Continental Polar and Maritime Polar Air Masses

Concept Check

What causes large amounts of snow to fall on the southern and eastern shores of the Great Lakes?

Continental polar air masses, crossing the Great Lakes, cause heavy lake-effect snows.

Assignment

Read Chapter 20, Section 1 (pg. 558-563)Do Chapter 20 Assessment #1-37 (pg. 583-

584)