Download Air Masses and Fronts

Chapter 3
Section 1
Air Masses and
Fronts
Four Major Types of Air Masses
in North America
Maritime Tropical
Maritime Polar
Continental Tropical
Continental Polar
Maritime Tropical
 Warm, humid air masses form over oceans near
the tropics
 Influence weather in the central & eastern United
States when formed over the Gulf of Mexico & the
Atlantic Ocean
 Influence weather on the West Coast when formed
over the Pacific Ocean
 Summer – hot, humid weather
 Winter – humid air can bring heavy rain or snow
Maritime Polar
 Cool, humid air masses form over the icy
cold North Pacific & North Atlantic oceans
 Affect the West Coast more than the East
Coast
 In summer, often bring fog, rain, & cool
temperatures to the West Coast
Continental Tropical
 Hot, dry air masses form only in summer
over dry areas of the Southwest & northern
Mexico
 Covers a smaller area than other air masses
 Move northeast, brings hot, dry weather to
the southern Great Plains
Continental Polar
 Large masses that form over central & northern
Canada & Alaska
 Brings cool or cold air
 In winter, bring clear, cold, dry air to much of North
America
 Can bring bitterly cold weather with very low
humidity.
 In summer, storms may occur when air masses
move south & meet maritime tropical air masses
moving north
Four Types of Fronts
Cold Fronts
Warm Fronts
Stationary Fronts
Occluded Fronts
Cold Fronts
 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 then pushed upward resulting in a
cold front
 Cold fronts move quickly & can cause quick
changes in the weather, including violent
thunderstorms
 Once a cold front moves through an area, cool,
dry air moves in and often brings clear skies &
cooler temperatures
Warm Fronts
 A moving warm air mass collides with a slowly moving cold
air mass
 As in a cold front, the warm air rises above the cold air
 If the warm air is humid, showers & light rain fall along the
front when the two air masses meet
 If the warm air is dry, scattered clouds form
 Warm fronts move much slower than cold fronts, therefore,
the weather may be rainy or foggy for several days
 After a warm front passes through an area, warm & humid
weather will follow
 In winter, warm fronts bring snow
Stationary Fronts
 Sometimes cold & warm air masses meet,
but neither has enough force to move the
other; the result a stationary front
 When the two air masses meet, water vapor
in the warm air condenses into rain, snow,
fog, or clouds
 If the front is stalled, the weather may be
days of clouds & precipitation
Occluded Fronts
 The most complex of the four fronts
 At an occluded front, a warm air mass is caught
between two cooler air masses
 The two cooler air masses meet & may mix
 Temperature near the ground becomes cooler
 The warm air mass is cut off (occluded) from the
ground
 As the warm air cools & its water vapor
condenses, the weather may turn cloudy & rainy
or snowy
Cyclones
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An area of relatively low air pressure is called a cyclone
Low air pressure is represented on a weather map as a L
Cyclone spin counterclockwise in the Northern Hemisphere
Cyclones play a large part in the weather of the United
States
 As air rises in a cyclone, the air cools, forming clouds &
precipitation
 Cyclones & decreasing air pressure are associated with
storms & precipitation
Anticyclones
 Opposite of cyclones in most ways
 Anticyclones are high-pressure centers of dry air
 High pressure areas are represented on a weather map as
aH
 Anticyclones spin clockwise in the Northern Hemisphere
 Cool air moves downward from higher in the troposphere
 As the cool air falls, it warms up, so its relative humidity
drops
 Descending air in an anticyclone causes dry, clear weather