1918-19.] The Cooling of the Soil at Night. 
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VI. The Effect of Re-aeration of the Soil after Rain. 
A remarkable sudden alteration in underground temperature — due, it 
would appear, to the re-aeration of saturated soil after rain — may be noted 
here. On December 5, 1918, heavy rain began at 10 a.m., when the 
surface was at 39° F. and the 6 in. depth 44° F. As the rain percolated 
through the colder surface layers the 6 in. temperature fell to 42° F. 
By 12.30 p.m. the ground was saturated with water standing in pools on 
the surface. Then the clouds cleared very rapidly, the sun shone brilliantly, 
and the air temperature rose to 52° F. 
As the warm air was drawn into the soil after the receding water the 
6 in. temperature rose to 46 ’5° F. — a rise of 4'5° F. in two hours. 
The reverse took place on December 10, 1918, when heavy rain began 
to fall at 2 p.m., when the surface was at 43° F. and the 6 in. temperature 
at 41 '5° F. The rain percolating through the warm surface soon raised the 
6 in. temperature to 43° F. also. By 6.30 p.m. the ground was flooded as 
before, by 7 p.m. the sky was clear and the air temperature had fallen to 
33° F. 
The cold air drawn into the soil by the receding water cooled the 6 in. 
temperature to 39° F. — a fall of 4’0° F. in three hours. 
I venture to think that similar results on a smaller scale must always 
take place when a sudden rise in pressure is associated with a sudden 
fall in temperature ; more especially when the rise in pressure and fall in 
temperature are in conjunction with the dropping of a strong wind, which 
would have tended to draw air out of the soil, and so make it necessary for 
more air than usual to enter to restore equilibrium. 
VII. The Thermal Effect of screening the Soil from Radiation, 
Evaporation, and Cold Precipitation. 
During the last four months I have made a series of observations on the 
minimum temperature of soil over which I had erected a canvas shelter, in 
the form of a small ridge tent, the sides of which could be opened and 
rolled up at will. 
The shelter has been kept open during the day, except in times of 
cold rain, sleet, or snow, or strong winds ; it has been shut regularly at 
night. 
The soil underneath has thus been more or less effectively sheltered 
from radiation, evaporation, and cold precipitation, whilst it has received 
insolation during the day and warmth from warm rains and wind. 
