THE ATMOSPHERE, WEATHER, AND CLIMATE 



19 



AIR MASSES 



A large area with essentially homogeneous 

 topography generally has characteristic climatic 

 features, especially pressure, temperature, and 

 humidity. These conditions are said to characterize 

 a climatic rhythm. A single climatic rhythm is the re- 

 sult of a single air mass or definite seasonal changes 

 in air masses. Therefore, an air mass is any extensive 

 portion of the atmosphere characterized by predict- 

 able climatic features. 



There are various levels of air mass classification. 

 The tu'o primary categories, polar and tropical, are 

 based upon the source of the air masses. The second- 

 ary subdivisions are maritime and continental and are 

 based on known differences in temperature, moisture, 

 pressure, and other atmospheric conditions. Fur- 

 ther subdivision is on the basis of geographic lo- 

 cations having further unique aspects of climate. 



The most important air masses in North America are 

 classified below (Figure 2.7). 



NORTH AMERICAN AIR MASSES 



Polar C'ontinental, or Polar Canadian, originates in 

 Alaska, northern Canada, and the Arctic Region. It 

 consists of cold, dry air that comes down through 

 Canada to afifect the midwestern and eastern United 

 States. In the winter, it brings cold and snow; in 

 the summer, fair weather. 



Polar Maritime is of two types. Polar Pacific origi- 

 nates in the North Pacific Ocean. It brings cool, 

 fairly moist air from the northern Pacific to cause 

 frequent fog and winter rains along the Pacific Coast. 

 After passing over the western mountain ranges, this 

 air mass warms and dries, becoming the Chinook 

 winds of the continental interior. Polar Atlantic origi- 



Figure 2.7 Air masses: cA, Continental Arctic; cP Continental Polar, a source of very cold winters; 

 mPP, Maritime Polar Pacific, a source of cool moist winds; mJP, Maritime Tropical Pacific, o winter 

 source of worm moist winds; cT, Continental Tropical, a source of dry hot winds; m7G, Maritime 

 Tropical Gulf, a source of worm moist winds; mJA, Moritime Tropical Atlantic, o source of warm moist 

 winds; mPA, Maritime Polar Atlantic, a source of cool moist winds. 



