70 



from the sea and is therefore strongly influenced by continental con- 

 ditions. To what degree is the marine influence shown? 



Meteorologists (cf. Fassig, '99) have come to look upon the large 

 areas of permanent high and low barometric pressure as among the 

 most important factors in climatic control. There are five of these 

 powerful "centers of action" which influence our North American 

 climate (Fig. i), and four of these are at sea. A pair of loius are in 

 the far north, one in the north Pacific near Alaska, the other in the 



Fig. 1. DiagTam showing the positions of the relatively stable areas of high and 

 low barometic pressure, and indicating their influences upon the evaporating power of 

 the air and upon the climate in general. 



north Atlantic south of Greenland. A pair of highs are farther south, 

 one in the Pacific between California and the Hawaiian Islands, and 

 the other centering in the Atlantic near the Azores. The highs and 

 lows in each ocean seem to be paired and to have some reciprocal rela- 

 tion. The fifth center of action is upon the land. It is a high baromet- 

 ric area in the Mackenzie basin of Canada, where it becomes a pow- 

 erful center of influence through winter and spring, but with the prog- 

 ress of summer conditions weakens, and through the accumulation of 

 continental heat becomes converted into a lozv; thus there is a complete 

 seasonal inversion on the continent. 



These large highs and lows, although relatively permanent, are con- 

 tinually changing in intensity and position. The highs are regions of 

 descending, diverging, warming, and drying air, producing clearing 

 and clear air on their western side, but the reverse on their eastern side. 



