You will recall a preceding statement that evaporated humidity turns
into water when it becomes cool below a certain point. (See page 14,
Effect of the Sun.) A given amount of air will hold a certain amount of
moisture. For example, let us assume that a cubic foot of air (see Fig.
11) is saturated, that is, it is holding all the water it will retain.
Now if this cubic foot of air is cooled, it will contract, and as a
result there will not be enough room to hold both the air and moisture,
so the excess moisture will leak out. (See Fig. 12.) The result of this
reduction in temperature causes precipitation, simply because the air
cannot sustain the water that is in it. Therefor, at any time when
moisture in the air has reached the point of saturation and a chilling
takes place, due to the air becoming cold, rain follows. This may happen
as a result of air rising into higher places or cooler levels, or
through its contact with cooler surfaces.