Download Chapter 13 Section 3

Chapter 13 Section 3
Air Masses and Fronts
Essential Questions Ch.13 S3
1. What are the major types of air
masses in North America? How
do they move?
2. . What are the main types of
fronts?
3. What type of weather is
associated with cyclones and
anticyclones?
Air Masses
 An air mass is an immense (huge) body
of air that is characterized by similar
temperatures and amounts of moisture at
any given altitude (height)
 Usually covers hundreds of thousands of
square miles (over millions of square
kilometers and can be up to 10
kilometers deep)
Classified according to 2
characteristics:
 Temperature
 Humidity
4 Major types of Air Masses that
influence the weather in North
America
 Maritime tropical  Maritime polar
 Continental
 Continental
tropical
polar
 The characteristics of an air mass
depend on the temperatures and
moisture content of the region over
which the air mass forms.
Temperature affects air pressure.
 Cold, dense air has a higher pressure ,
while warm, less dense air has lower
pressure.
 Tropical or warm air forms in the tropics
and has low pressure
 Polar or cold air mass forms north of
Whether an air mass is humid
(moist) or dry depends on if it forms
over land or water.
 Maritime air masses form over
oceans and the evaporated water
from the oceans makes the air
humid.
 Continental air masses form over
land and have less exposure to
moisture and therefore are dry.
Air Masses
Weather in North America
 Continental Polar Air Masses
• Continental polar air masses are uniformly
cold and dry in winter and cool and dry in
summer.Tropical Air Masses
 Maritime
• Maritime tropical air masses are warm,
loaded with moisture, and usually unstable.
• Maritime tropical air is the source of much, if
not most, of the precipitation received in the
eastern two-thirds of the United States.
Air Masses
Weather in North America
 Maritime Polar Air Masses
• Maritime polar air masses begin as cP air
masses in Siberia. The cold, dry continental
polar air changes into relatively mild, humid,
unstable maritime polar air during its long
journey across the North Pacific.
• Maritime polar air masses also originate in
the North Atlantic off the coast of eastern
Canada.
Air Masses
Weather in North America
 Continental Tropical Air Masses
• Only occasionally do cT air masses affect the
weather outside their source regions.
However, when a cT air mass moves from its
source region in the summer, it can cause
extremely hot, drought like conditions in the
Great Plains.of cT air masses in the fall result
• Movements
in mild weather in the Great Lakes region,
often called Indian summer.
How Air Masses Move
 When an air mass moves into an area and
interacts with other masses, it causes the
weather to change.
 In the continental United States, air masses
are commonly moved by the prevailing
westeriles and jet streams.
 The prevailing westerlies, the major wind
belt over the U.S., generally push air
masses from west to east.
 The jet streams are embedded in
the prevailing westerlies and are
bands of high speed winds, As the
jet streams blow from west to east,
air masses are carried along their
tracks.
Fronts
When two air masses meet, they form a
front, which is a boundary that separates
two air masses. Air masses don’t easily
mix.
Storms and changeable weather often
develop along fronts.
Types of Fronts
 Colliding air masses can form four types of
fronts: cold fronts, warm fronts,
stationary fronts, and occluded fronts.
 The kind of front that develops depends on
the characteristics of the air masses and
how they are moving.
Cold front
A cold front forms when cold, dense air
moves into a region occupied by warmer air.
Cold fronts tend to move quickly and can
cause abrupt weather changes (including
thunderstorms).
After a cold front passes through an
area, colder, drier air moves in. Can bring
clear skies, a shift in wind, and lower
temperatures.
Warm Front
A warm front forms when warm air moves
into an area formerly covered by cooler air.
Clouds and precipitation accompany warm
fronts
Weather may be rainy or cloudy for several
days
After a warm front passes through an area,
the weather is likely to be warm and humid.
Stationary Fronts
Occasionally, the flow of air on either
side of a front is neither toward the cold
air mass nor toward the warm air mass,
but almost parallel to the line of the front.
In such cases, the surface position of the
front does not move, and a stationary
front forms.
Neither one can move the other --a
“standoff”
If it remains stalled over an area, it may
bring days of clouds and precipitation.
Occluded Fronts
 The most complex weather situation
occurs at an occluded front, where a
warm air mass is caught between two
cooler air masses.
 The warm air mass is cut off (occluded)
from the ground.
 As the warm air cools and its water vapor
condense, the weather may turn cloudy
and rain or snow may fall.
Cyclones
The term cyclone, in common use, is
sometimes applied to a tornado. In the
science of meteorology, however, the
term has a different meaning. For
meteorologists, a cyclone — and its
counterpart, an anticyclone — is a largescale system of air circulation in the
atmosphere in the zones between the
equator and either of the poles.
Middle-latitude cyclones are large
centers of low pressure that generally
travel from west to east and cause
stormy weather.
 It can be considered as either producing
or resulting from differences in air
pressure in those zones. In a cyclone the
central air pressure is lower than that of
the surrounding environment, and the
flow of circulation is clockwise in the
Southern Hemisphere and
counterclockwise in the Northern
Hemisphere. Cyclones are also
characterized by low-level convergence
and ascending air within the system.
 Cyclones and decreasing air pressure are
associated with clouds, winds, and
precipitation.
Anticyclones
 An anticyclone system has
characteristics opposite to that of a
cyclone. That is, an anticyclone's central
air pressure is higher than that of its
surroundings, and the airflow is
counterclockwise in the Southern
Hemisphere and clockwise in the
Northern Hemisphere. Anticyclones are
usually characterized by low-level
divergence and subsiding air.
 The descending air in an anticyclone
generally causes dry, clear weather.
Review
1. What are the major types of air
masses in North America? How do
they move?
Maritime tropical– moves North
Continental polar – moves south and
collides with maritime tropical
Maritime polar – moves southeast
Continental tropical moves north and
occasionally northeast
2. What are the main types of
fronts?
Cold fronts
Warm fronts
Stationary fronts
Occluded Fronts
3. What type of weather is
associated
with cyclones and anticyclones?
Cyclone—clouds, wind, and
precipitation
Anticyclone – dry clear weather