Evening star. [volume] (Washington, D.C.) 1854-1972, 1931-07-23
1t Isn't the Heat, It's
Why Water Vapor Makes One Hot in Summer and
Cold in Winter Shown as Body's Reaction
to Atmospheric Satnration.

face and consequently on body Meals
we'll enough understood.
But the physics OF humidity, tne
special province of the Weather Bureau,
itself offers many delicate propiems ano
contradictions of the popular viewpoint.
There are three kinds OF humidity, Dr.
Humphreys explains, First is absolute
humidity-the amount Of water Vapor
per unit volume of space. as the NUMBER
of grams per cubic foot. This involves
the term "saturation point"-the state
of space containing all the water Vapor
possible at stated temperature IN the
presence of fiat surface OF pure wafer.
BY means of this concept 1t Is possible
to set up constants for various Cem-
peratures, which, of course, would not
ordinarily be reached except under ex-
perimental conditions.
But the humidity generally TAXED
about is relative humidity-the ratio OF
water vapor actually present IN a given
volume of space anywhere to the apSo-
lutte humidity of the same volume. Thlrty
per cent humidity, fer instance, Woulon
mean that the atmosphere was 81 per
cent water vapor. 1t would mean that
per cubic foot, for example, the actual
amount of water vapor present was 80
per cent of the greatest amount possible
in cubic foot at the same temperature
in the presence of fiat surface OF pure
water.
Then there is specific humidity, or the
actual weight OF water vapor present pr
unit weight of air. This differs essen
TALLY from the other two in that 1t deals
with volume of air rather than vol
ume of space. 1t is this distinction that
accounts for many of the misconcep-
tions regarding humidity.
Air Is Free Agent.
The air itself, DR.. Humphreys ex-
plains, has very little to do with hui
nudity There could be just as much
humidity if there were vacuum. 1t is
centrally supposed, Dr.. Humphreys
finds from the many inquiries made OF
the Weather Bureau. that the air itself
in some way Soaks up water like a
sponge until it gets as much as it can
hold. Actually what happens is that
the temperature at the surface Of the
water supplies, in some way, energy by
which a liquid is changed into gas
and this gas, in compliance with the gas
laws of physics, rises by virtue of the
heat energy pushing its molecules in all
directions 1t makes very little differ
erce what is present in the space into
which it arises. The actual condition
of dry air, DR.. Humphreys says, might
be compared to 10-acre field in which
OllC sllIdi OOy NdS playing. 1hC ISlcLLANS
space taken up by the boy would be
about the same as that taken up by the
dry air molecules Then the coming of
the water vapor particles would be COM
parable, so far as space is concerned,
to small puppy wandering into the
field.
Wet Air Lighter.
Water vapor is lighter than dry air.
1t consequently tends to go upward
and to expand in all directions, the
rate depending on such factors as
pressure. The hotter it is the more
water vapor is created, because of the
energy supplied for breaking loose the
gas molecules from the liquid Water
vapor constitutes only very small
fraction of the whole atmosphere, but
the theoretically possible saturation
point goes up very rapidly with heat.
At 90 degrees Fahrenheit it might con-
ceivably constitute 5 per cent by
volume, not weight, of the atmosphere
near the surface. At 40 Fahrenheit
it would drop down to less than l
per cent.
Fortunately for living things, the
greater the amount of water vapor
with increasing heat the more certain
is the automatic process by which the
condition is relieved Being lighter
than the rest of the air, it rises. con-
vection sets in and a thunderstorm
results This takes the water vapor
out of the air and the process starts
all over again. The more water vapor
the mere likely the thunderstorm. The
hotter it is the greater the amount
of water vapor. 1t is one of natures
automatic defense mechanisms.