1919-20.] Effect of Weather Changes on Soil Temperatures. 75 
15 growing days, although the mean temperature of No. 2 is higher 
than of No. 1. 
Thus not only the growing temperature of the plant, but the daily 
values that make up the mean, have to be known before we can make 
any rule for the behaviour of the plant under the given conditions, and 
the date when the minimum soil temperature at from 4 to 6 inches depth 
passes the growing temperature of the proposed crop is the real question 
that concerns the cultivator. 
If we take 5 5° C. as the average growing temperature of most crops, 
it is of interest to note how weather changes help forward or retard the 
arrival of this temperature in the soil at 4 inches depth. 
In March and April 1919 there was a succession of spells of very 
different weather, which showed by their influence on the 4-inch depth 
temperature that it is the overcast weather, with bright intervals and 
overcast nights, that most rapidly increases the temperature underground, 
and not, as might be expected, the bright, sunny weather with clear sky 
and low relative humidity. 
This is shown in Table XIII. 
Table XIII. — Types op Weather and the Arrival of Spring Underground. 
Date. 
Mean 4-incli 
Depth 
Temperature. 
No. of Days 
when Minimum 
4-inch Depth 
Temperature 
was above 5 '5° C. 
Type of Weather, 
1919. 
Mar. 21-26 
Fell from l - 9° C. 
0 
Snow showers, cold E. wind, clear 
Apr. 2-8 
to 0-6° C. 
Rose from 1*7° C . 
5 
nights with hard frost. 
Overcast, fair intervals, W. or S.W. 
„ 9-15 
to 7'8° C. 
Fell from 7 ’8° C. 
2 
wind, overcast nights. 
Clear sky, bright sunshine, clear 
„ 16-19 
to 4*5° 0. 
Rose from 4*5° C. 
4 
nights, N.W. wind. 
Overcast, fair intervals, overcast 
„ 20-23 
to 10 - 4° C. 
Fell from 10-4° G. 
1 
nights, W. or S.W. wind. 
Clear sky, bright sunshine, clear 
to 8*4° G. 
nights. 
i 
Thus, in 9 out of 11 overcast days, but in only 3 out of 11 clear days, 
the minimum temperature at the 4-inch depth was above 5*5° C. 
These figures emphasise the importance of the effect of frost and clear 
nights on underground temperatures; in fact, we may say that the soil 
temperature for any period in winter or spring is mainly dependent on 
the number of frosts which occur while the ground is open without deep- 
lying snow during that period. Frosts that occur when several inches of 
