PERMAFROST BLACK 285 



two or more years than is replaced. Hence a cold reserve is estab- 

 lished. 



Basically the process can be reduced to one of heat exchange be- 

 tween the sun, the atmosphere, and the earth. The sun, through solar 

 radiation (insolation) , and the interior of the earth, primarily through 

 conduction, supply practically all primary heat to the surface of the 

 earth (biological processes, natural or artificial fires, chemical re- 

 actions, cosmic or other radiations excepted) . This primary heat is 

 dissipated to the atmosphere and to outer space by conduction, radia- 

 tion, convection, and evaporation. The atmosphere, by warm winds 

 and precipitation, also distributes secondary heat to the surface of 

 smaller areas. 



We know that earth temperatures at the depth of seasonal change 

 are in most places within a few degrees of the mean annual air tem- 

 perature, and that a geothermal gradient is established from the sur- 

 face to the interior of the earth. The geothermal gradient at any one 

 place is relatively fixed from year to year, though it varies from place 

 to place and has changed markedly during geologic time. It is gen- 

 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. 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, and other factors. 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 dissipa- 

 tion of heat outweights input, a cold reserve is produced. If the cold 

 reserve remains below freezing for more than 2 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 evaluate climatic 

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

 heat or dissipation of heat (Lane, 1946, and others). Thus it is well 

 known that the following conditions tend to produce permafrost : 



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



2. Low precipitation the year around and especially low snowfall. 



3. Clear winters and cloudy summers. 



4. Rapid evaporation the year around. 



5. Strong, cold winds in summer and winter. 



6. Low insolation. 



