1919-20.] Effect of Weather Changes on Soil Temperatures. 69 
the normal. A similar result, though in a less degree, was noticeable in 
April 1911 and 1916. 
Some observations made in the spring and autumn of 1919 on the 
difference of temperature between the air and the surface under varying 
conditions of wind and humidity are given in Table IX, and it will he seen 
Table IX.-— Differences between Air and Surface Temperature in 
Winds of Varying Force and Humidity. 
7 a.m. 
1 
p.m. 
6 
p.m. 
it 
Fall of 
Fall of 
it 
Fall of 
Surface 
Surface 
+= 
Surface 
Date. 
Wind. 
below 
Wind. 
below 
Wind. 
below 
B 
Air Tem- 
Air Tem- 
5 
Air Tern 
& 
perature. 
a 
perature. 
a 
perature. 
1919. 
Apr. 
2 
W. 5 
65 
3*6° C. 
W. 4 
85 
1*3° C. 
55 
3 
0 
92 
ro°’c. 
E.N.E. 2 
92 
2-0° C. 
E. 2 
92 
•4° C. 
5 5 
4 
0 
98 
•2°C. 
W.S.W. 6 
65 
3-4° C. 
W.S.W. 6 
87 
•7°C. 
55 
5 
W.S.W. 6 
87 
•8° C. 
W.S.W. 6 
75 
2-0° C. 
W.S.W. 6 
75 
T6° 0. 
?? 
6 
W.S.W. 4 
87 
•5°C. 
W.S.W, 5 
65 
2’4° C. 
S.W. 4 
80 
•8° C. 
5? 
7 
S.W. 4 
75 
1-0° C. 
S.W. 4 
65 
1-6° C. 
S.W. 3 
65 
1’3° C. 
Oct. 
25 
N.E. 4 
75 
1*0° C. 
N.N.E. 1 
75 
1-2° C. 
W.S.W. 1 
75 
•9°C. 
55 
26 
N.W. 4 
75 
1*0° C. 
N.W. 6 
65 
3-2° C. 
N. 5 
75 
2-0° C. 
55 
28 
N. 4 
82 
•4° C. 
N.N.E. 4 
75 
2-3° C. 
N.E. 2 
92 
o-o° C. 
> 
: O 
& 
18 
W.N.W. 4 
82 
1-4° C. 
W.S.W. 6 
75 
2-5° C. 
S.W. 7 
82 
1*2° C. 
55 
21 
W.N.W. 4 
75 
1*7° C. 
N.W. 3 
65 
2-5° C. 
N. 3 
75 
D4°C. 
Dec. 
22 
W.S.W. 4 
92 
10° C. 
W. 8 
75 
2-2° C. 
W. 4 
82 
1*5° C. 
19 
N.W. 5 
55 
2-6° C. 
W.N.W. 5 
60 
2-5° C. 
W. 3 
90 
1-3° C. 
1920. 
Jan. 
7 
S.W. 3 
90 
•8°C. 
S.W. 6 
85 
1-8° C. 
S.W. 8 
80 
1-5° C. 
55 
27 
! 
S.W. 6 
' 
75 
21° C. 
S. 8 
72 
3'6° C. 
N.W. 6 
65 
2*2° C. 
that the differences vary between 0° C. and 3*6° C. under the extreme con- 
ditions of no wind and practically saturated air, and strong wind and dry 
air, respectively, A difference of 3*6° C. at the surface corresponds to a 
difference of about 1-5° C. at the 4-inch depth, showing how potent an 
influence wind and humidity are on underground temperatures. 
Section VII. — Frost. 
(a) The Effect of Frost 
At the beginning of a spell of frost, and before the surface actually 
becomes frozen, the fall in the underground temperature is of course fairly 
rapid, as it follows with normal amplitude and retardation the fall of the 
surface temperature. As soon as the surface freezes, however, this rapid 
fall is replaced by a slow, steady decrease in temperature, which is strongly 
in contrast to its former rapid fall. 
