260 black. PERMAFROST [Ch. 14 



erally considered as 1° F. for each 60 to 110 feet of depth in sedi- 

 mentary rock in the United States (Orstrand, 1939) ; possibly 0.1 to 

 0.2 calorie per square centimeter per day is transmitted to the surface 

 from the interior (Theis, unpublished manuscript). In contrast the 

 sun supplies possibly as much as several hundred calories per square 

 centimeter per day to the surface, depending primarily on the season 

 and secondarily on cloudiness, humidity, altitude, latitude, etc. This 

 period of rapid heating, however, is very short in the Arctic, and for 

 many months heat is dissipated to the atmosphere and outer space. 

 When dissipation of heat outweighs input, a cold reserve is produced. 

 If the cold reserve remains below freezing for more than two years, it is 

 called permafrost. 



Although the fundamental thesis of the problem is simple, its quanti- 

 tative solution is exceedingly complex. In only a few isolated areas 

 in the Arctic do we know anything of the geothermal gradients in and 

 below permafrost. The climate (including insolation) is so incom- 

 pletely known that at present it is not possible to rate climatic factors 

 except in a general way as they effect primary or secondary heat or 

 dissipation of heat (Lane, 1946). Thus it is well-known that the fol- 

 lowing conditions tend to produce permafrost: 



(1) Long, cold winters and short, cool summers. 



(2) Low precipitation the year around and especially low snow- 

 fall. 



(3) Clear winters and cloudy summers. 



(4) Rapid evaporation the year around. 



(5) Strong, cold winds in summer and winter. 



(6) Low insolation. 



The materials involved have different specific heats and different 

 heat conductivities (Shannon and Wells, 1947; Muller, 1945; W. O. 

 Smith, 1939, 1942). Chemical and physical properties vary widely, 

 yet are of primary importance (W. O. Smith, 1942; Taber, 1930a, b). 

 Water transmits heat about 25 times as fast as air, and ice 4 times as 

 fast as water. Thus, poorly drained silt and muck are much more 

 easily frozen than dry, coarse-grained gravel. W. 0. Smith (1942) 

 points out the marked effect of soil structures and of architecture of 

 pore space on thermal resistance in natural soils. 



The dissipating surface of the earth is even more complex and more 

 changeable. Water-saturated frozen vegetation and soil (bare of 

 snow) in winter can act as an active conductor, whereas lush, dry 

 vegetation and dry porous soil in summer is an excellent insulator. 



