CAUSES OF RAINFALL 69 



the Bay of Bengal, and at an altitude of 4,455 feet. Here the mean 

 annual rainfall is between 400 and 500 inches. Along the southern 

 base of the Himalayas it is 60 to 100 inches, on the bold western 

 coast 80 to 120 inches and over, and on the mountains of Burma 

 160 inches. (See map of Indian winds, Figs. 10 and 12.) The pre- 

 vailing direction of the rain-bringing monsoon of India is from 

 the southwest, that of the Pacific coast of Asia from the southeast. 

 In general, the eastern^ coasts in the tropics are the rainiest, though 

 there are notable exceptions to this rule. 



The westerly winds of both hemispheres (southwesterly in the 

 northern and northwesterly in the southern), which blow poleward 

 above the latitudes of 30°, have a corresponding effect on the rain- 

 fall, the precipitation occurring on the westerly side of the mountain 

 ranges, while corresponding dryness prevails on the eastern. In 

 general, bold west coasts on the polar side of latitude 40° are very 

 rainy, the precipitation being 100 inches or more a year in the most 

 favorable situations. The interior of the continents, on the other 

 hand, especially when well enclosed by mountains, or open only to 

 the cool ocean winds which become warmed as they proceed, have 

 only a slight rainfall of from 10 to 20 inches or less. The eastern 

 coasts are drier than the western, but wetter than the interiors of 

 the continents. The maximum rainfall occurs in winter over the 

 oceans, islands, and west coasts of the continents, this being the 

 season of strong westerly winds, when cyclonic storms are most 

 numerous and best developed. The cold lands, too, have a chilling 

 effect on the inflowing damp air. In the interior of the continents 

 high pressure prevails and the winds flow outward, while their 

 low temperature at the start makes them drying winds as they pro- 

 ceed outward to warmer regions. This season is, therefore, the 

 driest for the interior of the continents. 



The greater part of the United States lies within this zone of 

 prevailing westerlies, the direction for almost all the country being 

 from the southwest. In winter these winds from the Pacific yield 

 their moisture on reaching the cool lands, this occurring even at 

 low altitudes, as a result of which the lowlands of California have 

 a wet season. This condition also exists in summer in Washington 

 and Alaska, while southern California has its dry season. Rising 

 over the mountains back of the coast, the winds yield more mois- 

 ture, so that all the regions west of the crest of the coast ranges are 

 well supplied with rain and snow in winter. Beyond this range, and 

 beyond the Sierras and Cascade Mountains further north, the winds 

 descend and become warmer and drier, and here lie the scmiarid 

 lands of eastern Oregon and Washington, and the Great Basin with 



