1918-19.] The Cooling of the Soil at Night. 
125 
III. The Method of taking Observations. 
(a) Radiation and Relative Humidity. 
According to Angstrom, “ The cooling of a body, exposed to radiate to a 
clear night sky, is almost independent of the temperature of the surroundings, 
provided that the relative humidity keeps a constant value.” * 
From the observations made by Angstrom in Algeria and California in 
1912 and 1913, I have worked out the relation between the rate of radiation 
and the relative humidity at each of these observations. The curve 
obtained is given in fig. 2, and gives the rate of radiation in calories per 
square centimetre per minute for any given value of the relative humidity. 
RELATIVE 
110 120 130 140 150 160 170 180 190 -200 
RATE OF RADIATION 
CALORIES PER SO- CENTI M ETRE PER MINUTE. 
Fig. 2. 
By taking several observations of the value of the relative humidity 
during the night, a mean value for the whole night is obtained, and the 
corresponding value of the rate of radiation is read off this curve. 
I have employed the magnitude of the stars visible as a test of the 
complete clearness of the night; and I have only given the results of 
observations on nights of little or no wind, when stars of the 5th magnitude 
were clearly visible. 
The total radiation in calories for the period h is 60 R/i. 
( b ) Latent Heat of Freezing. 
From the table of constants the latent heat liberated on freezing 1 cubic 
centimetre of soil is 36 calories. Therefore the latent heat liberated when 
*“A Study of the Radiation of the Atmosphere,” by Anders Angstrom, Smithsonian 
Miscellaneous Collections , vol. lxv, No. 3. 
