1919-20.] Effect of Weather Changes on Soil Temperatures. 63 
values tend to return to normal so soon after rain has ceased that it seems 
reasonable to suppose that the percolation of water rather than a change in 
the conductivity of the soil is the cause of these fluctuations. 
Rain is thus a great equaliser of temperature between the surface and 
underground — in fact, it is probable that the first rise of temperature 
underground from the low level of winter is due to warm spring rains. It 
is interesting to note in this connection that the time of day when most 
rain falls at Kew Observatory is as follows : — * 
April-August : Noon-6 pan., when surface is warm. 
September-March : 6 pan -6 a.m., when surface is cold. 
(b) Rapid Changes in Underground Temperature after Heavy Rain. 
Given heavy rainfall, and a rapid subsequent change from cloudy to 
R 
clear sky, we should expect these exceptional values of ^ and the lag to 
Ro 
cause very marked rapid changes in underground temperature. For if 
the rain has been heavy enough to saturate the soil and form in pools on 
the surface, the surface water must be considerably warmed in the sun or 
cooled under the clear night sky, before it succeeds in draining away, and 
this heating or cooling will in turn be communicated to the underground 
layers through which it percolates. 
Moreover, the soil air which has been rapidly driven downwards before 
the percolating water must be renewed from the surface as soon as the 
rain ceases, and it too will be warmer or colder than the air whose place 
it takes, according to the time of day at which the sudden change from 
rain to clear sky takes place. 
Four instances of this exceptional result are given in Table V ; the 
Table V. — Rapid Changes of Underground Temperature after Heavy Rain. 
Date. 
Duration of Rain. 
Subsequent Change in Underground 
Temperature. 
1918. 
Dec. 5 
10 a.m.-12.30 p.m. 
6-inch depth temperature rose 2 ’5° C. in 2 hours. 
» io 
2-7 p.m. 
„ „ „ fell 2-2° C. 
1919. 
June 7 
5-6.30 p.m., very heavy. 
4-inch depth temperature fell 2*5° C. in 1 hour. 
„ 13 
1 
8 a.m.-12 noon. 
,, ,, ,, rose 3 0 C. ,, 
temperature curves of the surface and 4-inch depth temperatures on 
June 13, 1919 show the following remarkable features: — 
* Meteorological Glossary , 1918. M.O. 225, ii, p. 230. 
