Transcript Air Masses - Cloudfront.net

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 dry
Continental Tropical (cT) – warm and dry
Maritime Polar (mP) – cold and moist
Maritime 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. 583584)