60 
Proceedings of the Royal Society of Edinburgh. [Sess. 
On clear nights, when the surface of the soil is of much the same 
temperature as the air above it, some of this rising vapour must be trapped 
in the rough surface layer and condensed there : one has only to examine 
the surface on a frosty morning, and notice the hoar frost on the under side 
of every little earth clod, to see that this is so. 
Now this condensation of vapour in the surface must give rise to the 
liberation of a considerable amount of latent heat which will go towards 
balancing the outgoing radiation, and will prevent the surface falling to as 
low a temperature as it otherwise would have done ; we should thus expect 
R 
that, since R 0 was diminished, the value of on such nights would be 
R 
above the normal, and would be greater than the values of from 
R° 
minimum to maximum on either side of it. 
This is shown in Table II : the variation will, of course, be noticed 
Table II. — Relation of ^ to Normal on Clear Nights when Risen 
±v 0 
Vapour condenses in Surface. 
Date. 
Previous 
r 4 
R 0 
r 4 
po 
Maximum to 
Minimum on 
Clear Nights. 
Subsequent 
r 4 
R 0 
Fall of Surface 
Temperature 
below 4-inch 
Depth Tem- 
perature. 
1919. 
Apr. 12/13 
'22 
•28 
•23 
4*9° C. 
,, 19/20 
•40 
44 
•40 
7-0° C. 
„ 20/21 
•40 
•42 
•40 
6*0° C. 
May 1 /2 
•38 
•41 
•36 
6-3° C. 
„ 2/3 
•36 
•39 
•38 
5’5° C. 
Oct. 8/9 
•26 
•34 
•29 
5-5° C. 
„ 12/13 
•30 
•37 
•30 
5-3° C. 
„ 15/16 
•30 
•36 
33 
4-0° C. 
Averages 
•33 
•375 
•34 
5*6° C. 
only on nights when the surface temperature falls so much lower 
than the 4-inch depth temperature as to be below the dew point of the 
rising vapour. 
(c) A Dry Surface Mulch. 
Dry soil contains a large percentage of air, and as air is an even worse 
conductor than water, the conductivity of soil is increased by wetting, and 
is at its maximum just after rain. As soon as a period of settled dry 
R 
weather sets in, the value of falls from the normal value of *42 in wet 
