Air Masses
Source Regions:
-The areas that air masses form in are called source regions.
-Air masses take on the characteristics of the surface area they are generated in.
-A source region must have fairly uniform topography and they can’t straddle an ocean.
-The area must also be still enough for the air above it to take on the characteristics of the land.
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Types of Air Masses:
-Continental Arctic (cA)
-Continental Polar (cP)
-Continental Tropical (cT)
-Maritime Tropical (mT)
-Maritime Polar (mP)
-Maritime air masses have more water vapor.
-Continental air masses have less water vapor.
-Polar air masses have cooler temperatures.
-Tropical air masses have warmer temperatures.
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Continental Polar:
-This type of air mass is very stable, cold, and dry.
-The surface temperatures of this air mass during winter months are as cold, and sometimes colder than
upper air temperatures, which creates a very stable environment.
-Lake effect snow:
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The lakes stay relatively warm for longer periods of time, and during late fall and early winter, the
phenomenon of lake snow effect can take place, usually on the leeward side of large lakes.
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Occurs when a cold cP air mass moves over warm water.
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A 20 degree Celsius difference between the air temperature and water temperature is the general
rule of thumb for this occurrence.
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As the cold, dry, stable air mass passes over the warm water, it becomes heated from underneath.
The warmed lower layer of the air mass creates instability by rising and condensing. Condensation
is increased by the large temperature variation within the air mass and the increase in elevation
(decrease in temperature), once it reaches the shore. The air mass reaches the shore unstable, and
drops moisture in the form of snow within 50-70 miles of the shore.
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Buffalo, NY and Syracuse, NY are two of the snowiest metropolitan areas in North America
because of lake effect snow. Both of these cities are in the “Snow Belt” and both have a large fetch
(distance for air mass to travel over the lake) where they are located next to Lake Erie and Lake
Ontario.
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Maritime Tropical:
-The Gulf of Mexico, the Caribbean, and the tropical Pacific Ocean are source regions for this type of air
mass.
-This air mass is very warm and humid. It is the main contributor of moisture to mid-latitude climates.
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Maritime Polar:
-The source region for this type of air mass is from the North Atlantic.
-This type of air mass is very cold, but has a lot of moisture.
-During winter months, a low pressure system may grab hold of this air as it moves up the eastern sea
board, and produce heavy rain, sleet, and/or snow for the northeast United States. This is known as a
nor’easter.
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Continental Tropical:
-The source regions for this type of air mass is in northwestern Mexico and southwestern U.S., only
during summer months when temperatures are extreme.
-If these air masses move, they can cause long periods of hot and dry weather.
-The Oklahoma dust storms were the result of the movement of this type of air mass for an extended
period of time.
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Air Mass Modification:
-As an air mass moves away from its source region it influences what it moves over, but is also influenced
by the land it moves over. Ex. Colder air masses can warm, warmer air masses can cool.
-Air masses that pass over cooler land areas will be cooled at the lowest levels and stability will occur.
-When an air mass passes over relatively warmer land, the lower layers of that air mass will heat and
instability will occur, causing ascending air and cloud formation.
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