Ice in the Sea 



259 



Referring to the special regional distribution of the three different types of ice (polar 

 ice, pack ice and solid ice) the central area of the North Polar ice consists always of 

 pure polar ice (Smith, 1931); it is 3-3-5 m thick at the end of the winter and 2-2-5 ra 

 thick at the end of the summer. It covers about 70% of the entire Polar Basin, i.e. 

 5-2 million km^. It is usually a continuous layer, but especially towards the edges it is 

 split up by ice pressure into large ice fields and ice floes. This large polar ice cap is 

 closely confined to the 1000-800 m isobath and has a more or less elliptical shape 

 lying much nearer to the continental coast and coastal islands on the Greenland- 

 North American side than towards the coast between Spitzbergen and Alaska where 

 the broad Siberian Shelf lies between. The centre of the polar cap is often called the 

 "pole of inacessibihty" and is situated about 400 nautical miles north of Alaska. 



The maintenance of this polar ice cap represents a state of equilibrium with the total 

 annual growth. The total gain consists at first of an addition of ice from the surround- 

 ing pack ice zone due to freezing at the bottom layers of ice floes, secondly of snow 

 falls on the ice surface and the re-freezing of open spaces. The ice loss is caused by 



Northern limit of drifting fiores <i 



-- Northern limit of ice bergs 



Pack ice limit 



Fig. 116. Average extent of sea ice in the Southern Hemisphere; the dotted line .... gives 



the mean northern iceberg limit, the continuous line gives the northern limit of pack 



ice and the broken line the northern limit of drift ice. 



