NO. 17.] 



THE TEMPERATURE OF THE POLAR ICE. 



563 



ture on each side of the median zero-line, (3) the whole temperature-system 

 by means of isopleths. 



Comparing the computed temperatures of the surface of the ice with the 

 smoothed monthly temperatures of the air, we have 



The surface of the ice, in all months with the single exception of June, 

 is warmer than the air. The difference is greatest in December. The sur- 

 face of the ice, being covered, except during a short time in summer, with 

 snow, is protected from cooling by radiation upwards, and receives heat from 

 the underlying warmer layers. The upper free surface of the snow radiates 

 heat freely towards the sky and space, and loses heat in a greater proportion 

 than does its lower surface in contact with the ice, and the resulting thermal 

 state of the upper surface is the main factor for the determination of the 

 temperature of the air. This radiation goes on throughout the year. During 

 autumn and winter, and particularly in the dark season, the cooling by radia- 

 tion of heat from the surface of the earth is greater than the warming effect 

 of the radiation from the sun, or is the sole agent. In spring and summer 

 the radiation from the sun prevails over the radiation from the earth and the 

 temperature of the upper surface of the snow, and the temperature of the 

 air rises more quickly than that of the surface of the ice, which is prevented 

 by the snow-covering from receiving the full effect of the radiation of the sun- 

 The fact that the surface of the ice has a higher temperature than the air, 

 is thus in full accordance with the natural conditions of the arctic ocean. 



The theory 1 of the propagation of heat in a soil of constant thermo- 

 metrical heat-conductivity down to great depths requires that the different 

 depths have the same annual temperature as the surface, and that the loga- 

 rithmic decrements of the annual ranges of the temperature in the different 

 depths are constant, and equal to the constant retardation of the phase with 



1 J. HANN. Lehrbuch der Meteorologie, p. 737. 



