Ice in the Sea 257 



varies within the hmits 6 x 10-' and 9 X 10"*. These investigations showed the con- 

 siderable effect of the temperature on the hardness of the ice. The strength of ice is 

 very important in calculating the loads that can be put upon it. The following empirical 

 data may be given based on experience : freshwater ice 4 cm thick will carry a man, 

 from 10-12 cm thick a galloping horse, from 15 cm thick a heavy-loaded truck, and 

 over 45 cm thick a railway train. This question is also of importance for aircraft 

 landing on ice. Moskatov (see "Die Naturverhaltnisse des Sibirischen Seeweges" 

 ("Conditions along the Siberian Sea route"), Oberkom. Kriegsmarine, BerUn 1949, 

 p. 84) has given the following table for the minimum safety thickness of freshwater 

 ice for aircraft landings : 



Aircraft weight (tons) 

 Minimum thickness (cm) 



2 

 15 



5 

 24 



10 



32 



15 20 



39 45 



The strength of sea ice, and that of salt-free ice formed from sea ice due to a decaying 

 process of several years is considerably less than that of freshwater ice. To carry 

 the same load the ice in the centre of the Arctic basin must be two to three times 

 thicker. 



3. Ice Conditions and their Seasonal and Aperiodic Variations in Arctic and Ant- 

 arctic Regions 



(a) Ice Conditions of both Polar Caps 



In the Northern Hemisphere sea ice is largely confined to the Arctic Mediterranean, 

 the central basin of which is always covered by it. Figure 115 shows the general out- 

 lines of mean ice coverage in summer and winter (Budel, 1943, 1950). September is the 

 time of minimum extension in ice cover, and the ice is limited to the inner part of the 

 North Polar Basin, which at that time is most remote from the warm land masses. 

 This ice lasts throughout the summer and then extends again enormously during the 

 winter. Except in the area of Gulf Stream water it reaches everywhere to the northern 

 coasts of the continents and extends as long tongues of pack ice along the eastern 

 coasts of Greenland and Labrador. To this winter ice then adds the one-year-old 

 winter ice of the adjacent seas. In winter, of the total area of the North Polar Basin 

 (11-6 milhon km^) on an average 8-7 miUion km^, (or 75%) are covered by ice. If 

 the pole were surrounded by land with a circular area of 2-9 million km^ then the 

 above mentioned ice-coverage would extend southward everywhere to the 72-7° 

 parallel (thus everywhere 17-3° lat. distance from the pole). 



In the Southern Hemisphere, where the Antarctic land mass surrounds the South- 

 pole with a total area of 14-8 million km^, the ice-coverage is 29-0 million km^ and 

 for an even distribution would then reach northward to the 55-8° parallel. These 

 figures show the strong contrast in ice conditions between the two polar regions. 

 The ice covers 3-35% of the total Northern Hemisphere, but 11-30% of the total 

 Southern Hemisphere. 



In the Southern Hemisphere (see Fig, 1 1 6) the ice extends uniformly around the 

 central Antarctic continent, enclosing it on all sides, and the symmetric circumpolar 

 arrangement of the ocean surface and the ocean currents fix zonal drift ice limits 



