WATER MASSES AND CURRENTS OF THE OCEANS 221 



In winter the average thickness of the arctic pack ice is probably 

 3 to 4 m, and in summer 2 to 3 m, but under pressure ridges and great 

 hummocks the thickness is greater. Hummocked ice is often found 

 stranded where the depth is 8 to 9 m, and in exceptional localities where 

 strong tidal currents prevail, masses of piled-up ice have been found 

 stranded where the depth was 20 m. The ice passes through a regular 

 annual cycle. In summer, melting takes place for two or three months, 

 and on an average the upper 1 m of the ice melts. In winter, ice forms 

 on the underside of the floes, but the thicker the ice is, the slower the 

 freezing. The average thickness of the ice depends mainly upon the 

 rapidity of melting in summer and freezing in winter, and is therefore 

 determined by climatic factors. Near shore, river water and warm 

 offshore winds facilitate melting and the development of navigable lanes 

 of open water along the coasts. In recent years the USSR has been able 

 to take advantage of such lanes along the north coast of Siberia for 

 establishing shipping connections with the large Siberian rivers. Aerial 

 surveys of ice conditions have preceded the operations and ice breakers 

 have been used where necessary. 



Within the greater part of the Polar Sea the ice moves mainly under 

 the action of the winds, but it is also carried slowly by currents toward the 

 opening between Spitsbergen and Greenland. The speed of the current 

 increases when approaching the opening, and great masses of ice are 

 carried swiftly south by the East Greenland Current. Since 1894 detailed 

 information as to the extent of the sea ice in the Norwegian Sea and in the 

 Barents Sea has been annually compiled and published by the Danish 

 Meteorological Institute. 



The icebergs in the Arctic originate from glaciers, particularly on 

 Northern Land, Franz Josef's Land, Spitsbergen, and Greenland. No 

 icebergs are encountered in the Polar Sea except near Northern Land, 

 Franz Josef's Land, and Spitsbergen, because the Greenland glaciers do 

 not terminate in the Polar Sea. The glaciers on the first three islands are 

 small and produce only small icebergs. By far the greater number and all 

 large icebergs originate from the Greenland glaciers and are carried south 

 by the East Greenland and Labrador Currents. These icebergs are 

 generally of irregular shape, because the Greenland glaciers do not form 

 thick shelf ice comparable to that of the Antarctic, but terminate in fjords 

 where piece after piece breaks off as the glacier advances. Some fjords 

 are closed by shallow sills on which the bergs run aground. 



Many of the icebergs that are carried south by the East Greenland 

 Current disintegrate before they reach Cape Farewell, the southern cape 

 of Greenland, but others are carried around the south end of Greenland 

 and continue to the north along the west coast. They are joined by other 

 icebergs from the West Greenland glaciers and together with these are 

 finally carried south by the Labrador Current. The icebergs reach farth- 



