NO. 17.] 



THE TEMPERATURE OF THE POLAR ICE. 



567 



The average for the year gives the water warmer than the ice. In 

 August, September and October, the ice is warmer than the water, from 

 November to July, the water warmer than the ice. The total effect is that 

 the colder ice in the course of every year causes the water to freeze, and 

 thus adds to the thickness of the ice. 



On the other hand it may be remarked, that the temperature of the air 

 on the route of the Fram is normally, as we have already seen (p. 483), above 

 0° for 18 days, from the 6* to the 24"^ July. In the sunny season the radia- 

 tion from the sun cannot raise the temperature of the surface consisting 

 of frozen water above zero; and its power is consumed in melting snow and 

 ice. The air with a temperature higher than zero is derived from warmer 

 regions by winds. The maximum temperature of the surface is 0°. The 

 melting and evaporation of the ice at the surface at this time of the year 

 are reducing the thickness of the ice. The period of melting is short, and 

 it is during this period that the evaporation is effectual. As soon as the 

 snow has begun to cover the ice, the evaporation from the surface of the 

 snow begins, the surface of the ice being protected against evaporation by 

 the covering of snow. The reduction of the thickness of the ice from its upper 

 surface may be assumed to be comparatively slight. 



The isopleths on PI. X show tlie following march of the temperature of 

 the ice in the course of the year. 



During the autumn and the winter the surface is constantly radiating 

 heat into the atmosphere and space, and its temperature decreases. As the 

 lower layers of the ice have a higher temperature, increasing with the depth, 

 there is a current of heat from below upwards towards the surface. From 

 the beginning of November, the underlying water is warmer than the lower 

 surface of the ice, and the upward current is thereby increased. This excess 

 of the temperature of the water increases during the cooling of the ice from 

 above, and the heat from the water militates against the cooling, and prevents 

 the ice from reaching such low temperatures as it would do in consequence 

 of the cooling from above through a sheet of ice with a tiiickness equal to 



