286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



The materials involved have different specific heats and different 

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

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

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

 1930b). 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-gi-ained gravel. W. O. Smith 

 (1942) points out the marked effect of soil structures and of architec- 

 ture 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 is an active conductor, whereas lush dry vegetation 

 and dry porous soil in summer is an excellent insulator. Black-top 

 pavements are good conductors and heat absorbers in summer and 

 can destroy permafrost. An elevated and insulated building with 

 circulating air beneath may unbalance the thermal regime of the 

 ground toward pergelation. Heat conductivities of some earth mate- 

 rials under fixed laboratory conditions are known, but the quantitative 

 effect in nature of variable moisture conditions and of changing vege- 

 tation is not. Changes in the volume, composition, or temperature of 

 ground water or surface runoff have effects as yet little known quali- 

 tatively or quantitatively. 



All these factors must be considered to be in a delicate balance be- 

 tween freezing and thawing. It is to be emphasized that the thermal 

 regime is not uniform, but changes from hour to hour, day to day, 

 week to week, year to year, and cycle to cycle. Spexjifically we must 

 think in terms of geographic position, topography, lithology, structure, 

 and texture of soils and bedrock, hydrology, geothermal gradients, 

 thermal conductivities, vegetation, climate (temperature, precipita- 

 tion, cloudiness, wind, insolation, evaporation), and cultural features. 



What effect cosmic dust clouds, changes in carbon-dioxide con- 

 tent of the atmosphere, inclination of the earth's axis, eccentricity of 

 the earth's orbit, sunspots, etc., have on permafrost can be surmised 

 only as they affect insolation and dissipation of the earth's heat. 



PRACTICAL APPLICATION AND SOLUTION OF THE PROBLEMS 



In a permafrost area, it is imperative that the engineer have a com- 

 plete understanding of the extent, thickness, temperature, and char- 

 acter of the permafrost and its relation to its environment before con- 

 struction of any buildings, towers, roads, bridges, runways, railroads, 

 dams, reservoirs, telephone lines, utilidors. drainage ditches and pipes, 

 facilities for sewage disposal, establishments for ground-water supply, 

 excavations, foundation piles, or other structures. The practical im- 

 portance of the temperatures of permafrost cannot be overemphasized. 



