WINTER CONDITIONS 



In winter, that is from September to June, the arctic air masses 

 form over the polar basin: The continental type over the solid- 

 pack ice north of Bering Strait, an area about half the size of the 

 United States ; and the maritime arctic air over the Atlantic side 

 of the polar basin where a larger percentage of water exists and 

 where the water is relatively warm with respect to the sea-ice 

 present. The continental polar air masses come mainly from the 

 Siberian Anticyclone, which is a northern extension of the warm 

 central Asiatic Anticyclone, although distinctly not the warm type 

 itself. 



Unstable maritime polar air masses are formed in the North 

 Atlantic during the entire year. The front between these masses 

 and the continental polar or arctic air from Canada, Greenland, 

 or Scandinavia is subject to the greatest storm activity in the 

 Northern Hemisphere. Here is experienced the worst flying 

 weather to be found on any of the north polar routes, if not on 

 the whole earth. 



The more stable types of both polar and maritime air come from 

 middle latitudes (around 45°-55° N.) in North America or Cen- 

 tral Asia, the Atlantic or Pacific. Being warm-core the masses 

 tend to stagnate in the regions where they are formed so that the 

 invasion of these masses into the polar regions is rare. Whatever 

 air of this type existing in the arctic or subarctic regions comes 

 rather by a slow process of extension of the area of the warm 

 anticyclones from southerly regions into the higher latitudes, as in 

 the fall or "Indian Summer" in central Canada, or in Siberia. 



About the only type of Pacific air that is found in the Arctic, and 

 that only rarely when a deep cyclone is centered in the Bering 

 Sea, is mPW air from the northern extension of the Hawaiian 

 High, which pours across the high ranges of the Alaskan coast 

 into the interior valleys. Occasionally, the stable continental air 

 drains down from the plateaus of the Rockies or of Mongolia, 

 into the arctic prairies of Canada or Siberia for weeks at a time, 

 even in December or January. This condition gives perfect CAVU 

 weather over a vast area, with temperatures perhaps 20° to 30° 

 above normal, thus reversing the normal condition when midwin- 

 ter is a season of little flying activity. 



Stable maritime air sometimes persists for long periods along 

 the coasts. In such cases aviation is hampered by surface fogs 

 or stratus, although such weather may be ideal for long distance 



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