21 
to the ground is comparatively slight, owing to the slow conduc- 
tivity of the soil and the small specific heat of the atmosphere. This 
point has been carefully developed by Maurer, of Zurich (1885). 
But when rain and snow fall, then the latent heat formerly con- 
tained in the atmospheric vapor is quickly given to the surface soil 
and directly conducted deeper into the ground, and the latter is 
warmed or cooled according as the rain or snow is warmer or cooler 
than it. In general, the warming of the soil by warm rain is less 
important than the cooling by cold rains, melting snows, and evapo- 
rating winds. 
CLOUDINESS. 
When clouds intervene the soil receives a smaller proportion of 
direct solar heat, and the proportion diminishes as the thickness of 
the cloud layer increases or as the proportion of cloudy sky to clear 
sky increases. We may adopt the approximate rule that the warm- 
ing effect of the sunshine is inversely as the cloudiness of the sky 
within 45° of the zenith; thus for a sky covered by 10 cumulus or 
10 stratus the direct solar heat at the ground is 0; for 10 cirrus or 
cirro-cumulus or cirro-stratus the solar heat is about 5, while for 0 
cloudiness the radiation that the observer receives is 10. 
SOIL THERMOMETERS. 
The motions of the clouds do not affect the sum total of the 
intensity of the sunshine, but the variations of cloudiness are so 
important that it is best to make use of some form of sunshine 
recorder or, better still, some form of integrating actinometer as a 
means of determining the relative effectiveness of the sunshine for 
any hour or day. If any such instrument shows that during any 
given hour, with the sun at a known altitude, the duration of the 
effectiveness of the sunshine was the nth part of the maximum value 
for clear sky, then we may assume that the heating effect of the sun 
on the surface of the soil was the vth part of its maximum value 
and may-thus ascertain and, if need be, approximately compute the 
irregularities of the diurnal waves of heat that penetrate the soil. 
But these irregularities are directly shown by thermometers buried 
in the soil at different depths, and the observation of such soil ther- 
mometers is an essential item in the study of climate and vegetation. 
The absence of these observations has necessitated much labor in 
unsatisfactory efforts to obtain the approximate soil temperatures 
from the ordinary observations of air temperature, radiation ther- 
mometers, clouds and sunshine. 
Fortunately the agricultural experiment stations of the United 
States have begun the observation of soil temperatures as distin- 
