Chapter 14 — AIR MASSES AND FRONTS 



characteristics, from the surface up, and 

 becomes virtually homogeneous throughout, and 

 its properties become rather uniform at each 

 level. In the middle latitudes, the land and 

 sea areas with the associated steep latitudinal 

 temperature gradient are generally not homo- 

 geneous enough for source regions. These areas 

 act as transitional zones for air masses after 

 they have left their source regions. 



The source regions for the world's air 

 masses are shown in figure 14-1. Note the 

 uniformity of the underlying surfaces; also note 

 the relatively uniform climatic conditions in 

 the various source regions, such as the 

 southern North Atlantic and Pacific Oceans for 

 maritime tropical air and the deep interiors 

 of North America and Asia for continental polar 

 air. 



Arctic Air 



There is a permanent high-pressure area 

 in the vicinity of the North Pole, within which 

 is found the arctic air-mass source region. 

 In this region there is a gentle flow of air 

 over the polar icefields, allowing the formation 

 of the arctic air mass. The air is characterized 

 by being dry aloft and very cold and stable 

 in the lower altitudes. 



Antarctic Air 



This air is developed in the antarctic region. 

 It is colder at the surface and other levels 

 than arctic air in fall and winter. 



Continental Polar Air 



The continental polar source regions consist 

 of all the land areas dominated by the Canadian 

 and Siberian high-pressure cells. In the winter 

 these regions are covered by snow and ice. 

 Because of the intense cold and the absence 

 of water bodies, very little moisture is taken 

 into the air in these regions. Note that the 

 word "polar" when applied to air-mass 

 designations does not mean air at the poles 

 (this area is covered by the words arctic and 

 antarctic). Polar air is generally found between 

 40° and 60° latitude and except for that found 

 over northern and central Asia, is generally 

 warmer than arctic air. 



Maritime Polar Air 



The maritime polar source regions consist 

 of the open unfrozen polar sea areas in the 

 vicinity of 60° latitude, north and south. Such 

 areas are sources of moisture for polar air 

 masses; consequently, air masses forming over 

 these regions are moist, but the moisture is 

 sharply limited by the temperature. 



Continental Tropical Air 



The continental tropical source regions can 

 be any significant land areas lying in the 

 tropical regions, generally between 25° north 

 latitude and 25° south latitude. The large land 

 areas found there are usually desert regions, 

 such as the Sahara or Kalahari Deserts of 

 Africa, the Arabian Desert, and the interior 

 of Australia. The air over these land areas 

 is hot and dry. 



Maritime Tropical Air 



The maritime tropical source regions are 

 the large zones of tropical sea along the belt 

 of the subtropical anticyclones. High-pressure 

 cells stagnate in this area most of the year. 

 The air is warm due to low latitude and is 

 able to hold considerable moisture. 



Equatorial Air 



The equatorial source region is the area 

 from about 10° north to 10° south latitudes 

 within which the thermal Equator is found. It 

 is essentially an oceanic belt which is very 

 warm and has a high moisture content. 

 Convergence of the trade winds from both 

 hemispheres and the intense insolation over this 

 region causes lifting of the air, which is 

 unstable and moist, to high levels. The weather 

 associated with these conditions is characterized 

 by thunderstorms throughout the year. 



MODIFICATION 



When an air mass moves out of its source 

 region, there are a number of factors which 

 act upon the air mass to change its properties. 

 These modifying influences do not occur 

 separately. For instance, in the passage of 

 cold air over warmer water surfaces, there 

 is not only a release of heat to the air, but 

 also some moisture. 



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