where this type of fog is most likely to occur is the outer islands 

 of the Canadian archipelago, particularly Prince Patrick, Borden 

 and Meighen Islands. Here the prevailing winds are northwest 

 (from sea to land) at all seasons of the year. The fogs do not 

 penetrate far inland, and localities 20 miles from the coast are 

 relatively free of fog. Unlike other areas in the Arctic, the pe- 

 riods of maximum frequency of fog in the outer islands of the 

 Canadian archipelago would be spring and fall. 



The second type of fog of major importance in the Arctic are 

 the radiation fogs of winter. These fogs form readily under in- 

 version in very cold weather and are caused by the cooling of the 

 lowest layer of air in contact with the ground surface. Cooling 

 by radiation takes place most rapidly over the land areas and least 

 rapidly over open water. The fogs that are formed are generally 

 thin and shallow. They occur most frequently along river bottoms 

 where the air drainage is poor and the air movement sluggish. 

 The locale of most frequent occurrence appears to be in the lower 

 Lena River valley in Siberia and the lower Mackenzie River valley 

 in North America. They are also quite frequent in the Yukon 

 valley and the valleys of the principal northward-flowing rivers of 

 central and eastern Siberia. Radiation fogs are also common over 

 the pack ice during very cold weather. 



Over the river valleys and over the pack ice, these fogs may 

 be expected from 8 to 12 days monthly during the winter. In more 

 adverse years the number of days with radiation fogs may be 

 doubled. In a few localities, such as the lower Mackenzie in the 

 vicinity of Aklavik, early morning radiation fogs may be expected 

 20 to 25 days monthly during the coldest months. It should be 

 stressed that these frost fogs do not present as serious an obstacle 

 to aviation as do the ivater fogs. They are generally thin fogs, 

 so that contrasting objects can be distinguished on the surface 

 directly beneath the airplane. It is only in the brief interval when 

 the airplane is passing through the thin fog layer that visibility is 

 seriously reduced. With experience, landings and take-offs can 

 be made with safety through the shallow frost-fogs. 



Of considerable interest but of minor importance is the occur- 

 rence of steam fogs, also known as arctic smoke. They are formed 

 by steaming from open water surface during extremely cold 

 weather, occurring most frequently over rivers, unfrozen lakes, 

 and open leads in the arctic seas. Such steam fogs are generally 

 shallow and are quickly dispersed by wind. However, they may at 

 times be sufficiently dense to obscure landing fields adjacent to 



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