252 



Ice in the Sea 



and the result of this analysis, given in the last lines of Table 97, shows how regular 

 is the annual temperature wave, with a decrease in amplitude and a phase shift in the 



Table 97. Annual temperature variation at different depths in sea ice 

 (According to the values of the "Maud" Expedition, North Siberian Shelf) 



extremes, penetrating into the ice (Fig. 112). In both series of recordings there is good 

 agreement in the upper layers of ice down to about 1 -5 m, but this is not true at greater 

 depths. The "Fram" values are too low, probably due to the observational method 

 using bar-thermometers. The decrease in the annual amplitude with depth shows the 

 same. According to the "Maud" values the annual variation disappears at a depth of 

 2-9 m. At a depth of 2-8 m the temperature of sea-water underneath the ice floe 

 reaches — 1-6°C and remains constant throughout the whole year. At the side under- 

 neath an ice floe, the thickness of which varies on the average as seen from Table 97, 

 the amplitude of the annual temperature variation thus falls to zero. 



Fundamental investigations on the thermal conductivity of ice have also been made 

 by Malmgren. Stefan (1890) found a thermal conductivity coefficient k = 4-3 x 10~^ 

 from theoretical investigation of the process of ice formation, but this value can only 

 apply for freshly formed pure ice. Later Mohn (1 900) attempted to compute the thermal 

 conductivity coefficient from the decrease in the annual temperature variation and 

 from the retardation of the extremes with depth in ice ffoes. However, these methods 

 cannot give reliable values since the theory is valid only for infinite thickness, while the 

 thickness of sea ice is small and the lower side of a floe remains almost always at a 

 temperature of — I-6°C. Correct values of A' can be determined, according to Malm- 

 gren, from the temperature gradient and its change with time at diff'erent depths. 

 Assuming a cylinder with a vertical axis through an ice floe, then definite amounts of 

 heat will enter the cylinder through its upper surface in / sec. If the ice floe is of suflH- 

 cient horizontal extent no heat will pass through the vertical wall of the cylinder and 

 the heat flux will only occur normal to the surface of the ice floe. If the heat content 

 of the cylinder for a given time remains constant then kiGi = k^G^, where Ati, k^ and 



