Ice in the Sea 255 



temperature to very low values. The increase in temperature per day due to the flow 

 of heat Wa from below can be found from the mean height of this cold atmospheric 

 surface layer. As Malmgren showed, this heat is quite large and it is obviously this 

 source of heat that prevents an intensive cooling of the atmosphere above the North 

 Polar basin. The temperature can thus never reach the low values found in central 

 Siberia or central Greenland, where this heat source is not available. 



{d) The Mechanical Properties of Sea Ice 



The continuous formation of ice by freezing is counter-balanced by very effective 

 processes that reform and destroy the ice fields. The mechanical properties of ice 

 (elasticity, plasticity and resistance against deformation, bending and compression) 

 are of the greatest importance in the interplay between these processes. Large ice 

 surfaces seldom remain unchanged for longer periods. They are broken up rapidly 

 from the edges, by the combined action of the wind, waves and periodic tidal currents, 

 and in a short time become separate ice floes. With the aid of strong winds they are 

 piled up by the large horizontal pressures and pushed one above the other. The 

 resultant mass, when finally covered with snow, cemented together and built up into 

 several layers, is pack ice. Pressure and tensions are common in the polar regions 

 (especially in the Arctic). Gaps and open spaces may exist for a short time but are 

 rapidly covered over by young ice which again re-unites the whole mass. These 

 pressures are not due to the effect of the wind alone, because often the wind only 

 influences far-off regions, thereby subsequently causing pressures in the Arctic (distant 

 effect) ; they are often due to rapid temperature changes at the surface of the ice. Since 

 the under-side of an ice floe is always at the temperature of the water (near freezing 

 point) there will be tensions and stresses in the floe. Figure 1 14 shows schematically 

 the cracks and fissures formed when the stresses due to thermal expansion at the surface 

 exceed the elastic limit. In the same way thermal contraction at the surface forms in an 



Fig. 114. Changes in an ice floe due to thermally induced expansion. 



analogous manner cracks at the lower side. The cracks on the upper surface of the 

 floe soon fill with snow and melt water and those in the bottom surface fill with ice 

 due to the rapid freezing of sea water in contact with the cold ice. There is thus a 

 continuous formation of ice. The ice-covered regions in the Antarctic are not basins 

 surrounded by land, and therefore ice pressures occur less often and are considerably 

 weaker. The humps, hummocks and ridges of piled-up floes, known by the Siberian 

 name toross, which are sometimes up to 5 m or more in height are much less common 

 in the Antarctic pack ice; instead the action of pressure often forms folds and flexures. 

 The mechanical properties of ice, like its other properties, depend on the temperature 

 and salinity, but due to the multiplicity of ice forms and conditions these determine 

 only the order of magnitude, and there may be considerable variations caused by the 



