Chapter 16 Section 2:
Air Masses and Fronts
Air Masses:
Changes in the weather are caused by the movement of air masses. An air mass is a large
body of air with the same properties (temperature and pressure) as the area it formed over. These
areas are called source regions.
They are represented on maps with a two letter symbol. The first letter indicates the moisture
level of the air mass, and the second letter represents the temperature of the air mass. The map at
the left shows the source regions for the air masses that influence North America.
There are four source regions.
mT= Maritime Tropical-rainy, forms over the warm waters of the Tropics.
mP= Maritime Polar-snowy, forms over the cold waters of the polar regions.
cP = Continental Polar-dry, forms over land in the polar regions.
cT = Continental Tropical-hot and dry, forms over land in hot desert-like regions.
Cold Air Masses:
Most of our winter weather is caused by the polar air masses. Continental polar air from over
the middle of Canada, would cause extremely cold temperatures in the winter and cool, dry
conditions in the summer. Maritime polar air masses from over the Atlantic and Pacific, bring snow
in the winter and cooler, rainy weather in the summer.
Warm air masses:
Most of our summer time weather is influenced by the tropical air masses from the south.
Maritime tropical air masses from over the Gulf of Mexico, the Atlantic and Pacific Oceans, bring
warm temperatures, rain, and thunderstorms. The continental tropical air mass that forms over land
in Mexico and the deserts of the U.S., bring hot, dry conditions to the middle of the country.
Fronts:
Air masses do not usually mix. So, when two different air masses meet, a boundary is formed.
The boundary between two air masses is called a front. Weather at a front is usually cloudy and
stormy. There are four different fronts- Cold, Warm, Stationary, and Occluded.
When a cold air mass meets a warm air mass, a cold front forms. The cold air wedges under
and lifts the warm air upwards. Cold fronts can move fast, push the warm air up violently, and
produce thunderstorms. When a warm air mass meets and overrides a cold air mass, a warm front
forms. The warm air moves up and over the cold and generally brings drizzly precipitation. If a
fast-moving cold air mass overtakes a slower-moving warm front and then continues advancing and
catches another cold front, an occluded front forms. An occluded front has cool weather with a lot
of precipitation. When a cold air mass and a warm air mass parallel each other with little horizontal
movement, a stationary front forms. Weather is similar to a warm front, although it may possibly
last a longer time.
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Alto
Cold air
A fast moving cold air mass sharply lifts warm air.
A cold front is formed.
Warm air overrides cold air in a warm front.
Warm front
Warm front
When warm and cold air show parallel or little
horizontal movement, a stationary front forms.
Occluded fronts occur when cold or cool air
catches up to a warm front.
Questions Section 16-2: Answer the following questions searching the reading on the previous page.
__________1. Changes in the weather are caused by the movement of ___ ____.
__________2. An air mass is a large body of air with the same _____and pressure as the area it formed over.
__________3. There are ___ source regions.
__________4. Continental ____air is formed over the middle of Canada.
__________5. Maritime _____ air masses bring warm temperatures, rain, and thunderstorms.
__________6. When a cold air mass collides with a warm air mass, a ___ front forms.
__________7. When a warm air mass overrides a cold air mass, a ___ front forms.
__________8. The symbol for a maritime polar air mass is ___ .
__________9. There are ___ different kinds of fronts.
__________10. The boundary between two air masses is called a ____.
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