Table VI 

 Main Source Regions of Arctic Air Masses 



In summer the distinction between polar maritime, polar con- 

 tinental, and arctic air almost disappears due to the nearly uni- 

 form surface conditions over the arctic and subarctic. The frozen 

 ground over the continents thaws, at least at the surface, under 

 the influence of continual sunshine or daylight; the snow melts 

 from the glaciers and pack ice, the ice melts from the lake areas, 

 and the water area in the polar basin or Arctic Sea increases 

 markedly. Thus, the whole polar area becomes one of mild, 

 humid, semimaritime activity. Temperatures are uniformly be- 

 tween freezing and 50° F. Occasionally, higher temperatures 

 occur under the influence of strong importation from the south, 

 or of strong insolation in favored localities. 



Diurnal ranges, horizontal differences, and inter-diurnal varia- 

 bilities are slight. The latitudinal and continental influence is at 

 a minimum. Only the effect of elevation is increased due to the 

 greater humidity and lack of inversion, the former increasing the 

 lapse rate and blanketing the lowlands from excessive outgoing 

 radiation and the latter removing the anomaly, so common in the 

 arctic winter, of having pools of cold air in the hollows while hill 

 or mountain tops are 20°, 30°, and even 40° warmer. 



If any contrast exists in the Arctic in summer, it is between the 

 air over the pack ice at near 32° F. and the subarctic zone on the 

 one hand, and the air over the warm Atlantic water on the other 

 hand, where temperatures average 20° or 30° higher in July and 

 August. This contrast is destroyed frequently when small, but 

 sometimes intense, cyclonic storms invade the polar basin, tem- 

 porarily displacing the Arctic Front. Also, at any time during 

 the summer a mass of mAK air can build up some place in the 

 Arctic and move down over the Canadian islands or the Norwegian 

 and Barents Seas, giving summer snowstorms, freezing tempera- 

 tures, icing and fog to those parts. 



96 



