complicated than it looks
The problem of the moving air mass, however, is more
complicated than it looks. For with the air is mixed a quantity
of water vapor. In a strict sense they are independent
variables, and the view set forth in most text-books that air
has a certain capacity for water vapor is misleading. We seldom
meet with pure, dry air. A cubic meter of such a gas mixture
would weigh 1,247 grams, at a temperature of 283 degrees A. (50
degrees F.). If chilled ten degrees, that is, to the freezing
point of water, it would weigh 46 grams more. So that by
cooling, air becomes denser and heavier. A cubic meter of a
mixture of air and water vapor at saturation, at the first
temperature above mentioned weighs only 1,242 grams, or five
grams less, and if this were cooled ten degrees the mixture
would weigh three grams less than the same volume of pure dry
air. We see that in each case the mixture of air and water
vapor weighs less than the air by itself. One would think that
by adding water vapor which, while light, still has weight, the
total weight would be the sum of both. It really is so,
notwithstanding the above figures, and the explanation of the
puzzle is that there was an increase in pressure with
expansion, so that the volume of the air and saturated vapor
was greater than one cubic meter. Since then a cubic meter of
air and saturated vapor weighs less than a cubic meter of dry
air at freezing temperature, speaking generally, we may expect
moist air to rise and dry air to fall. Consequently, if in
addition to falling temperature there is also a drying of the
air, we shall have an accelerated settling or falling of cold
dry air to the ground, which of course favors the formation of
frost. The water vapor plays also another role besides that of
varying the weight per unit volume. The heat received by the
ground consists of waves of a certain wavelength; but the heat
re-radiated by the ground consists of waves of longer
wave-length, and these so-called long waves (12 thousandths of
a millimeter) are readily absorbed by water vapor. Thus water
vapor acts like a blanket and holds the heat, preventing loss
of heat by radiation to space. Further on we shall speak of the
high specific heat of both water and water vapor as compared
with air and show the bearing of this in frost fighting; but at
present we may from what precedes formulate the second law of
frost fighting as follows: 'Frost is more likely to occur where
the air is dry than where it is moist.' It is also true that a
dusty atmosphere is less favorable for frost than a dust-free
atmosphere. Thus we may generalize and say that whatever favors
clear, still, dry air favors frost. The theory of successful
frost fighting then is to interfere with or prevent these
processes which as we have seen facilitate cooling close to the
ground. In what way can this best be done?
