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Chapter 8: Air Masses
The Atmosphere:
An Introduction to
Meteorology, 12th
Lutgens • Tarbuck
Lectures by:
Heather Gallacher,
Cleveland State University
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What is an Air Mass?

An airmass is an immense body of air (1200+ km by
several km thick).

It has a similar temperatures and humidity
throughout.
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Source Regions

Source regions are where air masses originate.
 It must be an extensive and physically uniform area.
 The area is characterized by a general stagnation of
atmospheric circulation.
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Source Regions
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Classifying Air Masses

Classification depends on the latitude (temperature)
and the surface area and are identified by two-letter
codes.
 With latitude, air masses are in 1 of 3 categories.
 (P) polar
 (A) arctic
 (T) tropic
 There are two surface areas.
 (m) marine
 (c) continental
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Classifying Air Masses

cP = Continental polar—very cold and very dry

cA = Continental arctic—cold and dry

cT = Continental tropic—warm and dry

mA = Marine arctic—cold and wet

mT = Marine tropic—warm and wet
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Air-Mass Modification

When cA or cP air moves over the ocean in winter,
it undergoes significant changes.
 Evaporation from the water surface transfers moisture to
the dry continental air, leading to instability.
 When an air mass is colder than the surface, k can be
added to the air mass symbol.
 If an air mass is warmer than the surface, w can be added.
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Properties of Air Masses

Continental polar (cP) and continental Arctic (cA)
air masses:
 During the winter, air masses are dry and bitterly cold.
 Stable air is the rule because the air is very cold and the
surface below is frozen.
 During the summer, air masses are warmer and moister.
 Air is warmed from the ground and the higher Sun angle.
 It is still colder and drier than mT air.
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Properties of Air Masses
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Properties of Air Masses

Lake-effect snow is the result of a cold air mass
over warm water.
 The phenomenon occurs mainly in autumn and early
winter.
 cP air moves across open water (Great Lakes).
 Air increases temperature slightly from the warmer water.
 It picks up more moisture.
 cP air moves over land.
 It then cools and drops snow in large amounts.
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Properties of Air Masses

Lake-effect snow: Cold air over warm water
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Properties of Air Masses

Maritime polar (mP) air masses form over oceans at
high latitudes over the North Atlantic or North
Pacific.
 mP air is warmer than cP and cA masses in winter.
 mP air masses from the North Pacific influence North
American weather more than those generated in the northwest Atlantic.
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Properties of Air Masses

Pacific mP air masses usually begin as cP air in
Siberia.
 As the air moves east, it picks up moisture over the warm
ocean.
 When mP air arrives at the western coast of North
America, it is often accompanied by low clouds and
shower activity.
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Properties of Air Masses

mP air from North Atlantic was originally cP.
 It only occasionally affects weather on northeast coast of
North America.
 In winter, strong cyclonic winds can draw mP air from the
Atlantic into the region, which results in a Nor’easter with
strong northeast winds, freezing temperatures, high
humidity, and precipitation.
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Properties of Air Masses

Maritime tropical (mT) air masses most often
originate over the Gulf of Mexico, the Caribbean
Sea, or the western Atlantic.
 mT air masses are warm to hot, moist, and unstable.
 They are capable of significant precipitation.
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Properties of Air Masses

North Atlantic mT air masses from the GulfCaribbean-Atlantic source affect the weather of the
U.S. east of the Rocky Mountains.
 In the summer mT air masses affect a much wider area of
North America and are present for much of the winter.
 As it moves inward it becomes an mTk mass as daytime
heating increases instability.
 During winter mT air only occasionally enters the central
and eastern U.S.
 When the lower portions of air are chilled and stabilized,
mT air is changed to mTw with the possibility of fog and
increased precipitation.
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Properties of Air Masses

North Atlantic mT air
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Properties of Air Masses

North Pacific mT air masses from the Pacific source
have much less an impact on North American
weather.
 When the air mass moves northward, cooling causes the
lower layers to become more stable often resulting in fog,
drizzle, or moderate precipitation.
 mT air masses from the subtropical North Pacific can
cause the phenomenon known as the Pineapple Express
and bring extraordinary rains to southern California and
other West Coast locations.
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Properties of Air Masses

North Pacific mT air
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Properties of Air Masses

Continental tropical (cT) air masses:
 These air masses are usually produced in the interior of
Mexico and in the southwestern United States.
 cT masses are hot, dry air masses.
 It remains nearly cloudless with an almost complete lack
of rainfall, which can cause drought.
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End Of Chapter 8
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