TABLE 90. DECREASE IN THICKNESS AND PERCENTAGE OF ICE 

 COMPUTED FROM TABLES 88 AND 89 



Table 90 shows the decrease in percentage and thickness of ice, computed by use of tables 88 

 and 89, reckoning the thickness of ice on 21 May as equal to 200 cm and concentration equal to nine- 

 tenths. 



By comparison of the aforementioned tables there may clearly be seen the importance for the 

 melting of ice of the poljoiyas and tidal lanes whose water absorbs the heat and gives it up for m^elt- 

 ing of the ice. Thus, if there is nine-tenths ice 200 cm thick on 21 May with an average heat absorp- 

 tion by the water equal to 300 gram-calories per square centimeter per day, ignoring the melting of 

 ice from above and below, the percentage will decrease to one-tenth by 1 September; i.e. , in 100 

 days (as found from table 89). At the same time, if on 21 May we have ten-tenths ice, then by 1 

 September this ice will maintain its original concentration, and only its thickness will decrease 

 (as found from table 88) from 200 cm to 100 cm. 



It must be noted that table 88 is computed only for close, continuous ten-tenths ice. Table 89 

 presupposes that all the heat of solar radiation and of the atmosphere which is absorbed by the water 

 is expended only in reducing the horizontal dimensions of the open ice. In actuality, with open ice 

 theheatabsorbedby water is used not only in reduction of horizontal dimensions but also in reduction 

 of thickness of the ice. Thus, we have seen that even when the ice and water temperatures are the 

 same, owing to the different values of vapor pressure over water and over ice, there cannot be an 

 equilibrium in the atmosphere. A continuous evaporation will go on over the water, and over the 

 ice a continuous condensation of vapor, with a corresponding transfer of heat of evaporation from 

 water to ice. 



This circumstance is not taken into consideration in the aforementioned equations and tables. 

 The percentage of ice, entering into equation (3) as initial data, does not take into consideration the 

 dimensions of floes. It naturally follows that nine-tenths brash ice melts away and is destroyed 

 considerably faster than ice of the same percentage and thickness but consisting of large floes and 

 fields, since with volume being the same, the area subject to action of radiation and heat from the 

 atmosphere and water is considerably greater. 



LITERATURE: 68, 77. 



Section 118. Centers of Break-up and Melting 



By approximate calculation, during the daylight season of the year at 70° north, the amount of 

 direct and diffused solar radiation which reaches the sea surface is approximately 30 kg-cal per 

 square cm (taking into consideration the cloudiness and transparency of the atmosphere). At 80° 

 north the quantity of heat which reaches the sea surface is reduced to 20 kg-cal per square cm. 



319 



