Chapter 3: Weather Patterns
Section 1: Air Masses and Fronts
Types of Air Masses:
Tropical: warm air masses in the tropics, low air pressure
Polar: cold air masses in the north, high air pressure
Maritime: air masses that form over oceans, humid air
Continental: air masses that form over land, dry air
** Four major types of air masses influence the weather in North America: maritime tropical,
continental tropical, maritime polar and continental polar. See chart!!
Fronts: the area where two air masses meet and do not mix
** There are four types of fronts: cold, warm, stationary and occluded
Cold Fronts:
*all fronts occur at a line
*Forms cumulonimbus clouds
cold air
warm air
cold air smashes into warm air- warm
air pushes up quickly- cools, condenses
and forms clouds
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Cold Front: forms when cold air moves under warm air-forcing warm air to rise
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moves quickly
causes severe weather (thunderstorms)
the warmer (more humid) the warm air is and the colder (dryer) the cold air is the
more severe the weather.
Warm Fronts
*all fronts occur at a line
*Gradual movement of air
warm air
cold air
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Warm Front: forms when warm air moves over cold air gradually pushing warm air up
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moves slowly
less severe the weather/ longer periods of rain or snow
Stationary Fronts
*all fronts occur at a line
*air masses move together
cold air
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warm air
Stationary Front: forms when cold and warm air masses meet but neither has enough force
to move the other
moves very slowly
causes cloudy, rainy weather for long periods of time
not strong enough to take over the other-moves together
Occluded Fronts
warm air
*all fronts occur at a line
*Most complex Front
cold air
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cool air
warm air mass stuck between a cold
and a cool air mass- warm air is
squeezed up- cools, condenses and
forms clouds
Occluded Front: forms when a warm air mass is stuck between a cold and cool air mass
Most complex front
Moisture rises, cools, condenses and forms clouds
Produces rain or snow
Cyclones and Anticyclones:
Cyclone:
 Swirling center of low pressure
 Indicates rain, snow or cloudy weather
 Spins counterclockwise
 A hurricane is an entire low pressure system
Anticyclone:
 Swirling center of high pressure
 Indicates fair weather, little clouds
 Spins clockwise