SOIL HEAT 



323 



The upper soil layers vary in accordance with the air 

 temperature, the maximum and the minimum occurring 

 in the same month. A lagging (see Fig. 53) is apparent 

 in the subsoil, due to the slow response of this area to the 

 heat penetrating from above. These figures also show 

 the surface soil to be warmer in spring and summer, and 

 cooler in winter and fall, than the lower depths. The 

 surface soil not only never falls as low in temperature as 

 the air, but reaches a higher point in summer. This is 

 shown in the range of the air and soil temperatures. The 

 range for the air is 51.8°, while that for the soil is 57.8°, 

 56.3°, 53.8°, 51.0°, 45.5°, 38.7°, and 34.3°, respectively, 

 for the depths ranging from 1 inch to 36 inches. While 

 this range of soil temperature is greater in the aggregate 

 than that of the air, the changes are much slower and 

 often extend over a number of days, while the air may vary 

 many degrees in an hour. 



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Fig. 53. — Curves showing the average monthly temperature readings at 

 various soil depths. Average of twelve years, Lincoln, Nebraska. 



