736 STATE BOARD OF AGlllCULTUKE. 



TUIO DAILY REVERSION OF TEMPERATUREi AND THE TEMPERATURE GRADIBINT AT 



THE ADJACENT DEPTHS. 



Unless the soil is frozen it almost always has a gradient of temperature 

 at its adjacent depths. This gradient of temperature, however reverses 

 itself between day and night during the warm part of the year to the 

 depth that the diurnal-nocturnal amplitude of oscillation of tempera- 

 ture extends. During the day it decreases from the surface downward 

 and during the night the reverse is true. There are periods, however, 

 both in the night and day, when this gradient is not regular. This occurs 

 usually at the time that the reversion takes place. The magnitude of 

 the gradient between the different depths varies with the time, both at 

 day and night, and the air temperaure. 



This gradient of soil temperature at the adjacent depths and its re- 

 version between day and night is due, of course, to the fact that the air 

 temperature reverses itself during these periods and the soil receives or 

 loses its heat more at the surface, at any given time, than at the increas- 

 ing depths. 



The soil, down to the depth that the diumal-nocturnal amplitude of 

 oscillation of temperature extends, receives its temperature in the form 

 of waves. Thus, for instance, in the morning before sunrise, there 

 occurs at its surface a minimum temperature. At this time the tem- 

 perature is lowest at the surface and increases Avith depth. As soon as 

 the air temperature begins to rise, however, the temperature at the sur- 

 face of the soil also commences to climb. The minimum temperature, 

 however, is travelling downward as a wave, and lowers the temperature 

 of the various depths as it reaches them. But as soon as the tempera- 

 ture begins to rise at the surface, it also is conducted downward as a 

 wave and immediately follows the minimum temperature wave, and causes 

 a rise in temperature. At about this point a reversion takes place at and 

 near the surface and the gradient becomes discontinuous. The soil then 

 at the upper and lower depths has a higher temperature than at the 

 intermediate. 



The intensity of the minimum wave becomes less and less as it travels 

 downward until finally it reaches a depth where its influence is no longer 

 perceptible. The warm wave, which immediately follows it causes then 

 the temperature of all the depths to rise. A gradient then is again estab- 

 lished and decreases in magnitude from the surface downward. The 

 magnitude of this gradient between the adjacent depths increases, as 

 the temperature rises in all depths, until the maximum temperature is 

 attained at the surface when it becomes greatest. At this time the sur- 

 face soil possesses a very high temperature and the temperature falls 

 very rapidly with the increase in depth. 



The maximum temperature which occurred at the surface travels to 

 the lower depths as a maximum wave, and imparts to the different depths 

 a maximum temperature at different times. 



But this maximum wave is immediately followed by a cold wave which 

 lowers the temperature at the various depths and decreases the magni- 

 tude of the gradient. The cold waves of different intensities continue to 

 travel through the soil, and to cause a reversion of its temperature in 

 the same manner as the warm waves already discussed. 



A concrete illustration of the propogation of these cold and warm 



