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Unit 4-4:
Air Masses
What is an Air Mass?
• An air mass is a large section of the lower
troposphere.
• It is defined as an area that has nearly the
same temperature and humidity all
throughout.
• It must have the same temperature and
humidity conditions for several days before
it can become classified as an air mass.
What is an Air Mass?
• Air masses are very large.
• 2-3 will cover all of the continental United
States
• For Example:
– An air mass over the Gulf of Mexico will be
warm and humid.
– An air mass over Canada will be cold and dry.
Types of Air Masses
• Continental Tropical: cT
– Air mass that forms over a warm continent.
The air is warm and dry.
– Examples: Air masses forming over Mexico,
Central Africa, India, or South America.
• Continental Polar: cP
– Air mass that forms over a cold continent.
The air is cold and dry.
– Examples: Air masses forming over Canada,
Siberia, or Europe.
Types of Air Masses
• Maritime Tropical: mT
– Forms over a tropical ocean. The air in this
mass is warm and humid.
– Examples: air masses over the Gulf of
Mexico, central Atlantic, or central Pacific.
• Maritime Polar: mP
– Forms over a cold ocean. The air in this mass
is cold and humid.
– Examples: air masses over the arctic ocean,
northern Atlantic, or northern Pacific.
Weather and Air Masses
• Weather in an air mass is determined by
where the air mass originated.
– Ex: An air mass that is cP will carry with it
cold and dry weather.
• The air mass will slowly change as it
moves to a new area. This will result in a
slow change of weather.
Weather and Air Masses
• The conditions of an air mass depend on
one thing: The temperature of the area
beneath them.
• As the temperature conditions change, the
cloud formation changes.
– The type of surface, whether it is maritime or
continental, also has an effect.
Cloud Formation
• If the ground surface is colder than where
the air mass originated:
– Condensation forms which results in dew, fog,
and stratiform clouds.
– An inversion will form as a result of this new
temperature condition.
• Remember an inversion is where the air is colder
at the surface and raises in temperature in the
troposphere instead of decreasing.
Cloud Formation
• If the ground surface is warmer than
where the air mass originated:
– Warming causes convection and winds. This
forms cumulus clouds.
– Depending on humidity:
• If dry, then the weather stays relatively fair.
• If humid, then the cumulus clouds form
cumulonimbus clouds. This also occurs if the
surface the air mass moves over is water (a lake or
the ocean).
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