40 POLAR PROBLEMS 



highest pressure in the eastern hemisphere Hes over northeastern 

 Asia, indicating prevaiHng southerly and southwesterly winds along 

 the northern coast of Asia and Europe; hence it follows that the low 

 temperature of these northern coasts results from the extreme cold 

 due to radiation over the wide land area of Asia. 



Arctic Influences on the Pressure Movements over 

 North America 



A map of the pressure distribution over North America shows 

 that northern Canada lies between two great centers of cyclonic 

 action, one of them situated over the North Atlantic and centered 

 near Iceland, the other over the North Pacific to the westward of 

 Alaska. The winter mean pressure over northern North America is 

 then high, and the prevailing winds of the Arctic coast line are north- 

 westerly. 



The northern middle latitudes include a zone of peculiarly intense 

 cyclonic action. The normally deep depression over the North Pacific 

 is the resultant of a succession of cyclonic areas passing from the 

 western portion of the ocean with increasing intensity towards 

 the coast of America, where they seem to disperse,. but with strong 

 indications that other depressions born of the Pacific parent con- 

 tinue on across the continent. Towards the Atlantic development 

 proceeds apace, and a succession of deep cyclonic areas passing over 

 or near Newfoundland reach the North Atlantic, the resultant effect 

 being the normal deep Atlantic low pressure south and east of Green- 

 land. 



Meanwhile, as depressions pass across America, they are sometimes 

 followed by a wave of high pressure which can be traced from within 

 the Arctic Circle or at times would appear to be an offshoot from the 

 Asiatic high pressure (Figs. i-8). 



These high pressures are usually accompanied by very low tem- 

 peratures and, as they spread southward over Canada and the United 

 States, cause periods of bitterly cold weather which may extend even 

 to the Gulf of Mexico (Figs. 5-8). In some winters, especially those 

 when the western portion of the continent is abnormally mild, the 

 anticyclonic areas develop farther to the eastward north of Hudson 

 Bay and spread southward (Figs. 5-8). These are the seasons which 

 are particularly stormy in the Atlantic trade routes. 



In the warmer seasons of the year, the distribution of pressure and 

 the prevailing winds indicate a marked lessening of Arctic influences — 

 even very remote — over North America, except in the extreme north 

 and northeast. Cyclonic areas are feeble, and it is seldom that 

 anticyclonic areas that affect southern Canada and the United States 

 can be traced back to the Arctic Circle. Such anticvclonic areas 



