158 RELATIONS OF SOIL TO HEAT 



The results at Tubingen show that in perfectly fine 

 weather the surface of the soil reached a midday temperature 

 of 120, or more, from April to September. The highest soil 

 temperature was generally reached in July, the average for 

 that month being 1463. Taking the mean for the whole 

 year, the midday temperature at the surface of the soil in 

 fine weather is seen to be 48- 8 above the temperature of the 

 air. 



The daily determinations at Geneva, made in all kinds of 

 weather, show that the average midday temperature of the 

 surface soil is about 30 above that of the air in June, July 

 and August, and that at 3 inches below the surface the 

 midday temperature is in the same months io-i3 above 

 that of the air. For the whole year, the average midday 

 temperature of the surface is i7-4, and at 3 inches below the 

 surface 6-3 above that of the air. 



The minimum temperatures reached by the surface of a bare, 

 dry soil, freely exposed to the sky, are, on the other hand, 

 considerably below the minimum temperatures of the air. 

 These low temperatures of the soil are the result of the loss 

 of heat by radiation, and occur in the night or early morning 

 when the air is clear, and the sky free from cloud. A ther- 

 mometer lying on the ground, in an open place, will usually 

 show a lower temperature each 24 hours than a thermometer 

 a few feet above the surface. 



When a soil is shaded, or protected by any covering, its 

 range of temperature is much diminished ; it receives less 

 heat from the sun during the day, and it loses less by 

 radiation during the night. A mulching of straw or manure 

 will have the greatest influence in this direction. A layer 

 of snow is also very effective in preventing the extreme 



