centers will invade the region usually occupied by the Polar Anti- 

 cyclone with attendant gales and stormy weather. Incursions 

 such as these would be more numerous in the summer when the 

 anticyclone is weakest, and when it has been depleted by mass 

 outbreaks of cold air into lower latitudes. 



As we mentioned earlier, areas of low pressure pass along the 

 periphery of the cold air mass and certain areas are zones of great- 

 est frequency. One such zone lies along the Atlantic Arctic Front. 

 In this zone there is no well-pronounced prevailing direction and 

 winds are strong and variable. This front marks the mean south- 

 ern boundary of the arctic air mass. It extends from the vicinity 

 of southern Greenland northeastward into the Barents Sea and 

 beyond, changing its mean position with the progression of the 

 seasons. Another such active region exists along the Pacific 

 Arctic Front which extends from Alaska to eastern Siberia, pass- 

 ing through the Bering Sea. In addition there are two regions 

 with winds of monsoon character, one in Siberia and the other in 

 Alaska. Of local consequence are katabatic winds. These winds 

 are caused by the downrush of cold air from higher elevations. 

 The passage of a low may provide the necessary pressure gradient 

 to set such winds into motion. These winds f jrequently reach gale 

 force. If the area is snow-covered, the turbulent action of the 

 winds will carry the snow dust aloft to such heights as to blot out 

 the sun. Winds of this character can occur along the entire outer 

 portion of the arctic basin, where there are elevations or moun- 

 tains from which the cold air can sweep. They are known to 

 occur along the deeply indented coast of Norway, at Wrangel 

 Island, and in the Aleutians. 



Local winds in the Alaska region are known as ivilliwaivs, takas, 

 or kniks. Their strength is greatest in the months when the 

 temperature contrast between the land areas and the open water 

 is the largest. The critical velocity marking the beginning of 

 sufficient turbulence to carry the snow dust aloft is about 11 m. p. h. 

 At about 18 m. p. h. the turbulence is great enough to carry suf- 

 ficient snow dust aloft to reduce the visibility to a point presenting 

 a landing hazard to aircraft. 



REGIONAL WIND SYSTEMS 



The first region to be discussed is that portion of Siberia east 

 of the Verkhoyansk Mountains. The wind system of this region 

 is monsoonal in character. During the colder months, from Sep- 

 tember through March, a ridge of high pressure is present over 



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