79 
1919-20.] Effect of Weather Changes on Soil Temperatures. 
of the diffusivity of the soil, are so dependent on the percolation of rain, 
it is possible that the values commonly given for the diffusivity of the 
surface layers of the earth need revision. 
3. Underground temperatures are also considerably affected by — 
(а) Strong winds of low relative humidity. 
(б) The frequency and intensity of frost when the soil has no snow 
covering. 
(c) The depth of snow. 
(d) Weather changes of long period. 
4. The date of flowering of Coltsfoot appears to bear little relation to 
the monthly mean values of temperature, but is closely related to the 
number of frosts on open soil not covered with deep snow. It is possible 
that good results would be obtained by comparing the phenological 
returns of the last thirty years with the accumulated temperature under- 
ground above the growing temperature for each plant considered. 
{Issued separately March 25 , 1920 . ) 
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