174 RELATIONS OF SOIL TO HEAT 



respect ; a moist soil thus radiates heat much better than 

 a dry one. In an air-dry condition, only hygroscopic water 

 being present, the differences between the radiating power 

 of the different solid constituents are only slightly perceptible. 

 In a thoroughly moistened state the distinctions previously 

 observed disappear. All thoroughly moistened soils radiate 

 to nearly the same extent, and the rate of radiation is not 

 increased if the soils are saturated with water. The radiation 

 of heat by a moist soil is nearly equal to that shown by soot. 



Ahr points out that the radiating power of a soil by no 

 means determines its rate of cooling, this is largely deter- 

 mined by its specific heat, and its conductivity. A high 

 specific heat will tend to slow cooling. High conductivity 

 will favour a more rapid emission of heat, but a relatively 

 slow cooling of the surface, owing to the passage of heat 

 to the surface from the interior. 



Influence of Water on Soil Temperature. We have in 

 the three preceding sections found abundant evidence of the 

 leading part played by water in determining the relations 

 of soil to heat. The action of water in the soil is to diminish 

 its summer temperature. From its very high specific heat, 

 a wet soil shows little rise in temperature when exposed 

 to sunshine. The better conduction of heat in a wet soil 

 (wet peat excepted) tends also to equalize the temperature 

 of the surface and subsoil, and cause it more generally to 

 approach the mean annual temperature. The active radia- 

 tion of heat from a moist soil leads to loss of heat. The 

 evaporation of water from a wet surface has a still greater 

 effect in cooling the soil, owing to the large amount of heat 

 consumed in converting water into vapour (p. 107). King 

 has calculated that the evaporation of one pound of water 



