SOIL HEAT 



243 



tive of soil temperatures in temperate climates with moderate 

 rainfall : 



Table LI 



AVERAGE TEMPERATURE READINGS TAKEN AT LINCOLN, 

 NEBRASKA, 1890-1902. DEGREES FAHRENHEIT 



It is apparent that the seasonal variations of temperature 

 are considerable even at the lower depths. The surface layers 

 vary more or less in accord with the air temperature and, 

 therefore, exhibit a greater fluctuation than the subsoil. In 

 general, the surface soil is warmer in spring and summer than 

 the lower layers but cooler in fall and winter. The soil, on 

 the average, is warmer than the air in winter. This occurs 

 because the air responds more quickly to a change in solar 

 insolation than the soil. 



The curves showing the monthly march of soil temperature 

 at Lincoln, Nebraska (Fig. 45), reveal the lag of the tempera- 

 ture change in the subsoil due to slow heat penetration. It is 

 also noticeable that the monthly range in temperature change 

 in the surface soil is higher than that of the air. The abso- 

 lute range is, of course, greater for the air. It must be kept 

 in mind that changes in soil temperature are gradual, while 

 the air may vary many degrees in an hour. 



The daily and hourly temperature of the air and soil in 

 the temperate zone may show considerable agreement or 

 marked divergence according to whether the weather control 

 is cyclonic or solar. With solar control and a clear sky the 

 air temperature rises from morning to a maximum at about 



