1919-20.] Effect of Weather Changes on Soil Temperatures. 61 
soil to somewhere about ’28, the latter value depending on the length of 
the dry period and the depth of the mulch formed on the surface.* 
That this does not entail a decided check in the gradual warming up of 
the underground layers is due to the fact that a dry soil rises to a much 
higher maximum temperature than a wet one, for in the wet surface a 
considerable amount of the radiant energy of the sun is used up in 
evaporation rather than in raising the temperature of the surface. Whilst 
R 
a surface range of 20° C. in wet soil when ^=’40 produces a 4-inch depth 
K q 
R 
range of 8°C., a surface range of 27° C. in a dry soil when ^ = ‘30 will® 
produce a 4-inch depth range of 8*1° C., i.e. the temperature ranges at 
the 4-inch depth are practically the same. 
This automatic adjustment of the underground range is well seen in 
observations made during dry spells of 8 days in May 1919 and 16 days in 
May and June 1919, shown in Table III. 
Table III. — Ranges at Surface and 4-inch Depth during Dry Periods. 
Date. 
Number of 
Previous 
Dry Days. 
r 4 
R 0 ‘ 
Surface 
Range. 
4-inch 
Range. 
Hours of 
Sunshine. 
Remarks. 
1919. 
May 10 
0 
•40 
13*0° C. 
5-2° C. 
8*2 hrs. 
After 5 millimetres of rain 
„ 12 
2 
•36 
14'2° C. 
5-1° C. 
5-0 „ 
on May 18, -^ 4 rose to 
Ro 
„ 15 
5 
■32 
19-0° C. 
6-0° C. 
12-8 „ 
•43, and the surface range 
„ 17 
7 
•30 
20-0° C. 
6-0° C. 
7-0 „ 
fell to 18° C. 
May 18 
0 
•43 
18*0° C. 
7'8° C. 
7’0 hrs. 
On June 3, after the 16 dry 
„ 22 
4 
•35 
24-0° C. 
8-4° C. 
11-8 „ 
days a 1 y dry mulch 
had formed, but when 4 
„ 26 
8 
•30 
27-0° C. 
8T° C. 
11-4 „ 
millimetres of rain fell, 
» 28 
10 
•29 
28-0° C. 
8T° C. 
13-2 „ 
rose to *40, and the surface 
Jane 2 
15 
•28 
25-4° C. 
7-1° C. 
9"7 ,, 
range fell to 18 *5° C. 
It will thus be seen that, while a dry surface mulch has a decided effect 
R 
on the values of -p- and the conductivity of the soil, owing to the increased 
K 0 
range of the surface when the soil is dry, there is no great corresponding 
effect on the 4-inch depth temperature under a mulch of 1\ inches. The 
underground layers thus tend to rise fairly steadily in the spring and 
early summer, under normal weather conditions, in which deep mulches are 
not likely to form, so long as sunshine is not deficient. 
* “ The Cooling of the Soil at Night, with special reference to Spring Frosts,” by the 
Author, Proc. Roy. Soc. Edin ., vol. xl, 1919-20, part i, No. 2. 
