320 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



topics, this was the only meteorological fact necessary to be remembered 

 in connection with their discussions. Cold converts the moisture in the 

 atmosphere into snow, therefore cold regions will be regions of great snow- 

 fall, and the more intense the cold the more abundant the supply. Nothing 

 can be farther from the truth than this. The moisture must first be 

 present before it can be congealed ; and the evaporative agency, by means 

 of which it has been raised into the atmosphere, is heat. As already sutfi- 

 ciently explained in the preceding chapter, the greater the heat the greater 

 the amount of evaporation, other conditions remaining the same. The 

 moisture, once taken up, may be carried to a great distance before it becomes 

 condensed again, and whether it shall fall in the form of rain or in that of 

 snow depends on the direction and force of the air currents and the clima- 

 tological or topographical conditions prevailing in the region towards which 

 the moisture-laden air masses are borne. 



It has already been shown, with sufficient detail, how the exterior portions 

 of the continental masses receive more than their share of the water 

 abstracted from the ocjcan ; and how, in consequence of this, the interiors of 

 large land areas are comparatively dry. It has also been explained how it is 

 that very extensive regions unfavorably situated with regard to the prevail- 

 ing winds, and under the lee of high mountain ranges, may be thus rendered 

 so arid as to be almost uninhabitable. That which is true where the pre- 

 cipitation is all, or in large part, in the form of rain, is equally so where the 

 mean temperature is so low that snow predominates, ^'ast areas in Central 

 Asia, as we have seen, have a very low mean winter temperature, neverthe- 

 less they are but very scantily supplied with snow. The same is true of the 

 ranges of the Eocky Mountains and the Great Basin. The prevailing 

 moisture-beai'ing Avinds being westerly, the Sierra Nevada is the recipient 

 of a very large snow-fall ; while the ranges farther east, some of which are 

 nearly or quite as high as the Californian Sierra, receive but a small quantity 

 of moisture either in the form of rain or snow. Still some aqueous vapor 

 does get carried over into the Great Basin, and, becoming condensed into 

 snow on the summits of the ranges, remains during more or less of the sum- 

 mer, and Ijy its melting furnishes water enough to prevent the region from 

 being one of utter sterility. 



It is a clearly established fact, therefore, and abundant illustrations of it 

 have been furnished in the preceding pages, that cold alone is not sufficient 

 to bring about precipitation, whether it be in the form of rain or of snow ; 



