Chapter 14 — AIR MASSES AND FRONTS 



Winter Air Masses 



The following paragraphs primarily describe 

 winter air-mass weather of North America. 

 Significant features of air masses of the rest 

 of the world are also pointed out. 



CONTINENTAL ARCTIC (cA) . — Continental 

 arctic air, either k or w, is the coldest air 

 over North America; however, the cooling rarely 

 extends above 700 mb (10,000 ft). Continental 

 arctic air is designated k and has an unstable 

 lapse rate in the lower layers. The stability 

 of cA air, though, depends primarily on its 

 trajectory. If the path of cA air is cyclonic, 

 instability, snow flurries, and low cloudiness 

 result (especially in the Hudson Bay to Great 

 Lakes region). East of the Appalachians, cAk 

 air produces little weather. When cAk air has 

 an anticyclonic trajectory, the weather is fair 

 (as in the Midwest). 



Elsewhere in the world, continental arctic 

 air is significant only over western Europe 

 and the Antarctic Continent. The appearance of 

 cA over western Europe is infrequent; when 

 cA air does appear, it is heavily modified and 

 unstable, though quite cold. 



The Antarctic Continent is the spawning 

 ground for cA air in the Southern Hemisphere, 

 but this air seldom leaves that continent. When 

 it does leave, it rapidly becomes mP air. The 

 coldest air mass in the world is the antarctic 

 cA air mass. 



CONTINENTAL POLAR (cP). — When a cP 

 air mass moves out of its source region over 

 warmer land, the lower layers of the air are 



gradually heated and the stability is decreased. 

 As long as the air is moving over a snow- 

 covered surface, the decrease in stability 

 does not completely eliminate the stable 

 characteristics acquired in the higher levels 

 at the source region. Usually an outbreak of 

 continental polar air is accompanied by winds 

 of 15 knots or more, and this wind helps decrease 

 the stable conditions in the lower levels. If 

 this modified air moves rapidly over rough 

 terrain, the turbulence will result in low 

 stratocumulus clouds and occasional snow 

 flurries. (See fig, 14-2.) 



After the air passes over the snow-covered 

 regions and moves over a surface having a 

 temperature above freezing, rapid changes in 

 air properties normally occur. The surface 

 temperature increases rapidly and soon elimi- 

 nates the stable conditions that had existed. 



Since the heating from below is more rapid 

 than the addition of moisture, the relative 

 humidity is decreased. Under this condition, 

 the condensation level rises and skies are 

 generally clear. 



A particularly troublesome situation often 

 arises when the cold air flows from a cold, 

 snow-covered surface to a water surface, and 

 then over a cold, snow-covered surface again. 

 This frequently happens with air crossing the 

 Great Lakes. Air flowing over the water surface 

 is heated rapidly near the surface and may 

 eventually become unstable. Also, water vapor 

 is added quickly to the air by evaporation from 

 the relatively warm water surface. The air 

 becomes saturated, or nearly so. Water vapor 

 may be added to such an extent that steam fog 



«^i <<&> 



CLEAR AND COLD 



209.366 



Figure 14-2.— cP air moving southward. 

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