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nearest the ice will soon become saturated with water vapor. Evap- 

 oration will therefore depend on the diffusion of water vapor from the 

 surface and generally proceed at a relatively slow rate. Air currents 

 increase the rate of evaporation by inducing turbulent mixing of the 

 layers of air near the ice and by bringing new unsaturated air masses 

 from drier regions. A brisk wind, under conditions of low relative 

 humidity, may therefore result in the ablation of large quantities of 

 ice or snow even though the air temperature never reaches the melting 

 point and no melting occurs. 



MELTING 



Melting of ice, for the reasons already discussed, takes place mostly 

 at the expense of the heat of the surrounding water. This heat maj^ 

 have been absorbed from solar radiation in the vicinity, or provided 

 by currents originating in warmer latitudes. Melting also results 

 from direct absorption of radiation by the ice and from contact with 

 warm air. Ice will condense dew from warm, moist air on its surface, 

 and each increment of moisture so condensed will melt several times 

 its weight of ice in the ratio of the latent heat of evaporation to the 

 heat of fusion. 



Another factor tending to accelerate the rate of ice melting from 

 solar radiation, once it has commenced, is the increased stability of 

 the surface layers of the sea brought about by the freshening effect of 

 the melt water. Mixing between the surface and deeper layers, al- 

 ready diminished by the wave-damping action of floating ice, is 

 further decreased by the formation of a surface stratum of relatively 

 low density. The normal transfer to greater depths of heat received 

 as infra-red radiation in the top layers is retarded, and the melting 

 of the ice is thereby speeded up. 



The phenomenon of "'dead water" is sometimes encountered by ships 

 in areas where a layer of nearly fresh water derived from melting ice 

 extends to about keel depth. Under such conditions, the propulsive 

 power of the ship may be largely dissipated in generating internal 

 waves in the boundary between the fresher water and the more saline 

 water. The ship loses headway, answers her helm sluggishly, and 

 appears to be "stuck" in the water. Fortunately, this state of affairs 

 occurs only when the speed of the vessel is below the speed of propa- 

 gation of such waves, which is not more than 2 or 3 knots. "Dead 

 water" will therefore ordinarily affect only sailing vessels in light 

 winds, or tugs with very heavy tows. 



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