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In regions where the spring melting of ice is brought about chiefly 

 by atmospheric transfer of heat from lower latitudes, and where local 

 fogs restrict the solar radiation reaching the ice and sea surface, the 

 fresh surface layers of sea water may become greatly chilled, and 

 the rate of melting will be reduced. Here vertical exchange of water 

 caused by wind, sea, currents, and tides will contribute heat to the 

 upper layers and expedite clearing of the ice. An example of melting 

 due to tidal mixing occurs in the White Sea, where clearing of the 

 ice takes place rapidly in the spring through overturn of deeper water 

 with greater density than the surface layers but with temperature 

 above the freezing point. 



STAGES OF DISINTEGRATION 



In spring, as the duration of daylight begins to increase and the 

 mean air temperature at the sea surface rises, the snow cover of the 

 sea ice and the top layers of the ice itself begin to thaw. Under con- 

 ditions of low humidity, most loss on the upper surface of the ice will 

 take place through evaporation imperceptible to the ordinary ob- 

 server; where the relative humidity is higher, pools of dew and melt 

 water will form on the surface. This fresh water, running down 

 through cracks and holes in the ice, will freeze again on contact with 

 the cold sea water, thus sealing the openings. On the other hand, 

 cracks extended only part way through the ice will be widened by 

 the expansion of this water freezing in them, and even though plugged 

 at the top will now extend through to the water. On further rising 

 of the air temperature and melting of the surface, these cracks open 

 up again, and fresh water in a layer as much as 2 to 3 feet thick 

 flows under the ice. 



Sea ice less than a year old melts more readily than older ice because 

 of its higher salt content. Fast ice usually melts first near shore, 

 forming the so-called "offshore water." As melting progresses, the 

 ice farther out from shore becomes honeycombed with cracks caused 

 by tide, air temperature changes, temperature gradients in the ice, and 

 ice pressure. Under the influence of wind and current this ice now 

 commences to disintegrate. Then the increasing number of channels 

 and polynyas brings about the motion of the larger areas. With the 

 first strong wind these break into smaller pieces, and finally all the 

 fast ice passes over into pack ice. 



In the Antarctic, disintegration of pack ice is produced almost 

 entirely by the sea. There, surface pools are seldom seen on the floes 



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