218 



Physics of the Soil. 



263, Influence of Topography on Soil Temperature. The 

 degree of inclination of the land surface and the direction 

 of the slope, whether facing east, west, north or south, may 

 exert a marked influence upon the temperature of the soil 

 and particularly upon its diurnal range. The tempera- 

 ture of a stiff red clay soil, upon a level table, and upon a 

 south exposure sloping about 18, was found in the sur- 

 face three feet to be as represented in the table below : 



Showing the influence of topography upon soil temperature. 



KIND OF Soiii. 



DEPTH BELOW THE SDBFACE. 



1st foot. 2nd foot. 3rd foot. 



Red day, south slope ... 

 Bed olay, level surface. 



70.3 F. 

 67.2 



3.1 



68.1 P. 

 65.4 



2.7 ~~ 



66. 4 F. 

 63.6 



2.8 



Here it is seen that the effect of a south exposure is to 

 make a difference in temperature of from a little more than 

 3 F., in the surface foot, to a little less in the second and 

 third, feet. 



The reason for these differences will be readily under- 

 stood from a study of Fig. 61. Suppose A 6 5 B to rep- 

 resent a section of a prism 

 of sunshine falling upon 

 the hill A E B, where A E 

 is the south slope and E B 

 is the north. On account 

 of the sun not being di- 

 rectly vertical over the hill 

 ^\ fl the south slope receives as 

 -, much more heat in a unit 



FIG. 61. Influence of topography on soil 



temperature. ol time than the north 



slope as the line 4-6 is longer than the line 4-5. 



264. Influence of Looseness and TJnevenness of Surface on 

 Soil Temperature. When a field is left very uneven, and 



