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numerous. For example, assume a specified day had 8 hours below 

 freezing, and the mean temperature for those 8 hours was 25° F. The 

 degree-days of frost for that day are %4 x 7 = 21/3. Suppose the next 

 day had temperatures below freezing for the entire 24 hours, and the 

 mean temperature was 20° F. The degree-days of frost for this day 

 are 12. If the next 10 days had approximately the same mean tem- 

 perature, say —20° F., and all hours were below freezing, the degree- 

 days of frost for the 10 days are 520. The total degree-days of frost 

 for the 12-day period described above would be 5341/^. A glance at 

 figure 3 shows that the curve permits considerable approximation in 

 calculating degree-days of frost without seriously affecting the final 

 results. Obviously other factors such as wind, snow, and currents 

 introduce complications difficult to evaluate and not allowed for in 

 the graph. The lowermost end of the curve is none too reliable be- 

 cause freezing weather may exist for a number of days before ice 

 starts to form. In addition, the number of variable factors affecting 

 cooling, mentioned heretofore, is diificult to evaluate. Once a layer 

 of ice has begun to form, the curve is much more reliable. 



The annual history of ice in far northern harbors is shown in figure 

 4. The size of such curves will differ from place to place, but their 

 shape will undoubtedly be similar. The important things to note 

 are the steady increase in thickness for two-thirds to three-fourths of 

 the total period, the brief flattening off, and finally the sudden drop 

 at the end. 



COMPARISON OF ARCTIC AND ANTARCTIC ICE 



Differences in underlying factors specific to the region develop cor- 

 responding differences in the features of the ice. An example of one 

 of these agencies is the low mean annual temperature of the Antarctic. 

 The warmth of the Arctic summer has no parallel in the far South and, 

 mainly because of this thermal difference, the ice sheets of the northern 

 polar regions are unlike those of the southern. The margin of the 

 Antarctic cap, overflowing its land support, is free to spread over the 

 sea until fracture detaches huge strips, sometimes including 10 to 20 

 miles of its front. In Greenland, by contrast, the edge of the inland 

 ice ends on land, and icebergs irregular in shape are formed. The 

 tabular or box-shaped berg is, therefore, in general, characteristic of 

 the Antarctic while the pinnacled, picturesque berg is typical of the 

 North. 



The Antarctic sea ice surrounds the continent, while the Arctic sea 

 ice is a central mass surrounded by land. The ice moves around and 



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