50 DISCUSSIONS IN CLIMATOLOGY. 



heat received even less, than at the summits, the snow 

 melts and disappears. Here, again, the influence of that 

 potent agent, aqueous vapour, comes into play. At 

 high elevations the air is dry, and allows the heat 

 radiated from the snow to pass into space ; but at low 

 elevations a very considerable amount of the heat 

 radiated from the snow is absorbed by the aqueous 

 vapour which it encounters in passing through the 

 atmosphere. A considerable portion of the heat thus 

 absorbed by the vapour is radiated back on the snow ; 

 but the heat thus radiated being of the same quality as 

 that which the snow itself radiates, is on this account 

 absorbed by the snow. Little or none of it is reflected, 

 like that received from the sun. The consequence is, 

 that the heat thus absorbed accumulates in the snow 

 till melting takes place. Were the amount of aqueous 

 vapour possessed by the atmosphere sufficiently dimin- 

 ished, perpetual snow would cover our globe down to 

 the sea-shore. It is true that the air is warmer at the 

 lower than at the higher levels, and, by contact with 

 the snow, must tend to melt it more at the former than 

 at the latter position. But we must remember that the 

 air is warmer mainly in consequence of the influence 

 of aqueous vapour, and that, were the quantity of 

 vapour reduced to the amount in question, the differ- 

 ence of temperature at the two positions would not be 

 great. 



But it may be urged, as a further objection to the 

 foregoing conclusion, that, as a matter of fact, on great 

 mountain-chains the snow-line reaches to a lower level 

 on the side where the air is moist than on the opposite 

 side where it is dry and arid — as, for example, on the 

 southern side of the Himalayas and on the eastern side 

 of the Andes, where the snow-line descends 2000 or 

 3000 feet below that of the opposite or dry side. 



