19 
1919-20.] The Cooling of the Soil at Night. 
Therefore, the temperature of the surface can only fall 17 ‘1° from a 
maximum of 18° ; in other words, the minimum surface temperature will 
be *9° C. and there will be no frost. 
The temperature so obtained by the formula is, of course, the minimum 
temperature of the surface under ideal conditions of perfectly clear sky, 
no wind, and dry air. A slight breeze, a temporary clouding of the sky, 
or the formation of a heavy dew will prevent the surface falling to as low 
a temperature as will be expected from the forecast equation. This can 
be clearly seen in Table VIII. 
VII. A Comparison of the Forecasted and Observed Minimum 
Temperatures of Open Soil on Clear Nights. 
When the surface temperature of a soil of high-water content has 
fallen to 0° C., latent heat is liberated in freezing the surface layer, and 
the surface-soil temperature does not fall below 0° C. until the surface is 
completely frozen — a process that takes some considerable time. 
As the forecast equation does not take this liberation of latent heat 
into account, it is only useful to forecast the occurrence, not the intensity 
Table VIII. — Forecasted and Observed Minima. 
Date. 
(1) 
Mo- 
(2) 
m 4 . 
(3) 
r 4 
Ro 
(4) 
Estimated 
"of Relative 
N "' / Humidity. 
6 p.m. 
^ Forecast of 
3 Surface 
Minimum. 
Observed 
3 Surface 
w Minimum. 
Difference of 
'oo Columns 
(6) and (7). 
Weather. 
(9) 
1919. 
April 9/10 
14-0 
8-0 
*40 
per cent. 
90 
Frost 
0*0 
Clear, calm, 
„ 12/13 
16-6 
9-6 
•42 
90 
55 
+ 
•4° 
high barometer 
55 
„ 19/20 
15-0 
11*2 
•40 
75 
+ 0-6° 
+ 
0-6 
o-o° 
55 
„ 20/21 
18*0 
11*8 
•42 
75 
+ 0-4 
+ 
0-6 
- -2 
55 
May 18/19 
23-8 
16-5 
•43 
80 
+ 5*1 
+ 
5*7 
- *6 
55 
„ 19/20 
29-8 
18-6 
•40 
75 
+ 4-7 
+ 
4-4 
+ *3 
55 
„ 26/27 
30-8 
19-0 
•30 
75 
+ 4-7 
+ 
43 
+ -4 
55 
June 2/3 
28-2 
18-0 
•28 
80 
+ 4-2 
+ 
4-0 
+ -2 
55 
„ V8 
32*7 
22'2 
•42 
80 
+ 9*1 
+ 
91 
o-o 
55 
„ 16/17 
21*7 
169 
•40 
85 
+ 6-6 
+ 
7-0 
- -4 
55 
„ 25/26 
28*3 
17*6 
•42 
70 
+ 3*6 
+ 
3-8 
- -2 
55 
July 12/13 
29-0 
17-6 
•40 
70 
+ 4-6 
+ 
5-0 
- *4 
55 
„ 13/14 
28-3 
17-5 
'40 
70 
+ 3-6 
+ 
3-8 
- *2 
55 
Aug. 12/13 
333 
23 3 
•31 
85 
+ 10-5 
+ 11T 
- *6 
55 
21/22 
29-0 
18*0 
•42 
75 
+ 4-3 
+ 
4-0 
+ -3 
55 
Sept. 27/28 
10*2 
8*2 
•40 
80 
Frost 
o-o 
55 
Oct. 12/13 
10*2 
7-8 
•40 
80 
55 
0*0 
55 
„ 13/14 
7*1 
5*6 
•40 
75 
5; 
- 
•3 
55 
„ 14/15 
5 3 
4-6 
•40 
75 
55 
" 
•5 
1 
55 
Average difference (13 occasions), ‘3°. 
