THE SHIFTING OF CLIMATIC ZONES. 



191 



of the high degree to which it is warmed by the Gulf Stream, is a most important area of 

 permanent low pressure. To this is thought to be due in large measure the fact that the 

 northern United States and Canada on the west and northwestern Europe on the east, 

 together with the Atlantic Ocean between them, are the most stormy regions of the globe. 

 In summer, when the continents become warm and therefore are characterized by low 

 pressure, the North Atlantic low area loses its importance. The difference in pressure 

 between land and sea is slight, and the storms are correspondingly mild. They move 

 nearly from west to east, although of course with many curves, and their tracks are usually 

 located well up toward the north. In winter, on the contrary, the continents cool off and 

 become areas of pronounced high pressure, while the oceans are areas of low pressure. 

 At this time the difference in pressure between North America and the North Atlantic 

 reaches a maximum, the barometric gradients are steep, and storms are correspondingly 

 fierce. The courses of the storms under such conditions are more curved than in summer 

 and lie farther south. The center of the continent becomes so cold that an extensive 

 area of permanent high pressure is formed; from this the winds blow outward. Thus the 

 storms wliich would otherwise move more or less directly east from the Pacific to the 

 Atlantic are pushed in many cases far to the south. Starting in Cahfornia, a storm may 

 swing southeast into Arizona and Texas, and then move east and finallj' come up the 

 Atlantic coast and swing off toward the low center of the North Atlantic. In its wake such 

 a storm maj^ send the thermometer down to 20° F. in southern Arizona and kill the peach 

 blossoms which have opened too early; then it may go on to produce a "norther" with a 

 temperature of 50° in Yucatan, and to kill the orange trees in Florida. The number of 

 storms which follow such southerly courses varies greatly from year to year. Upon these 

 variations the character of the winter largely depends. In 1911-12, for example, the 

 northern parts of the United States had few storms and slight snowfall as a general rule 



12? lio^ up Tio^ io? ioo' 9? 9? i? 80' 7? 7& 6? io' 55" 



Fig. 51. — Storm Fre<iuenry, 1878-1887. (After Dunwoody.) 



