396 TROPICAL SNOW LINES. 



somewhere about 16,000 feet over sea level; but this 

 is liable to be modified by local causes that is to say 

 it rarely comes down much lower than that, but owing 

 to the set of the prevalent winds and the aspect of 

 the mountain slopes, it is sometimes pushed consider- 

 ably higher up, say to 17,000 or even 18,000 feet. Thus 

 on the Bolivian Andes the line of perpetual snow has been 

 fixed at 16,000 feet.* The mean result of Humboldt's 

 observations made in the equatorial regions of South 

 America a century ago, gave almost similar results. 

 He fixed it then at 15,748 feet. Furthermore on the 

 Himalayas, the Indian surveys show that on their 

 southern face the snow line ranges from 15,000 to 

 1 6,000 feet. This is mainly due to the influence of the 

 winds that come up from the plains, charged with 

 moisture, which is here precipitated in the form of snow ; 

 they then pass northwards, as dry cold air, mainly de- 

 prived of their moisture; with the result that on the 

 higher plateaux of the Indian watershed, the altitude 

 of the snow line rises to 18,500 feet, while on the 

 great table-land of Thibet on their northern or reverse 

 face it reaches about 20,000 feet, f This affords us an 

 admirable instance of the immense influence which high 

 mountains always exert upon climate. At these great 

 altitudes rain very generally falls in the form of snow. 

 When the monsoons burst in torrents of rain upon 

 the great plain of Northern India therefore, the water 

 falling upon the higher Himalayas, does so as snow 

 and the intensity of the cold created by the upcast 

 of the aereal currents into the higher regions of the 



* See Stanford's Compendium of Geography and Travel for Central 

 and South America, edited by H. W. Bates, 1878, p. 207. 



f See Encycl. Brit., Qth Edition, Vol. xi, p 83 1 (Article " Himalaya "). 



