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 CHAPTER III 



EVAPORATION, MELTING, AND BREAK-UP 



HEATING AGENTS 



Ice and snow are evaporated and melted by direct absorption of 

 radiation and by conduction of heat from the surrounding air, rocks, 

 or water. The ultimate source of the heat energy is the sun in either 

 case, but the relative importance of radiation and conduction in melt- 

 ing ice will vary with climatic conditions in different localities. 



A large part of the visible radiation striking a white surface such 

 as ice or snow is reflected away from the surface again without 

 warming it, just as a white uniform keeps the wearer cooler than a 

 dark one, even if both are made of the same material. The percentage 

 of incident light reflected from a surface is called the albedo. The 

 albedo of clean snow is about 80 percent, of sea ice about 50 percent, 

 and of sea water only 3 or 4 percent. In the case of the longer wave 

 lengths in the infra-red portion of the spectrum, which make up 

 slightly over half the total radiant energy received from the sun, the 

 proportion reflected by a snow surface is only 15 to 25 percent. The 

 proportions reflected by ice and water are correspondingly less, and 

 since water is opaque to infra-red radiation, the heat absorption by 

 water in this region of the spectrum is concentrated in the surface 

 layers where it is of most significance with regard to melting of ice. 



It is obvious, therefore, that a surface interrupted with areas of 

 water, either leads between the floes or pools of melt water accumulat- 

 ino- on top, will absorb much more radiant heat than a continuous 

 ice or snow surface. Once disintegration of an ice sheet has proceeded 

 to the point where free water surfaces appear, the rate of further 

 disintegration is very much accelerated. Likewise, lowering of the 

 albedo of the ice or snow through other causes, such as- accumulation 

 of dust or a film of diatoms, will speed up disintegration. At the 

 beginning of summer, ice generally disappears first in the coastal zone 

 where it has become dirty from the proximity to shore. 



EVAPORATION 



The absorption of heat by ice or snow results in either evaporation 

 or melting. Melting takes place as soon as the temperature of any 



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