The Cecil Whig. [volume] (Elkton, Md.) 1841-current, 1891-07-18
HOW RAIN Is FORMED.
Manner In Which the Thirsty Air Absorbs
ana Releases Moisture.

A correspondent asks for mforma-
tion as to the process by which the air
sucks up moisture from the surfaces of
oceans, lakes, rivers and moist earth,
and lets It fall again in the shape Of
rain. Perhaps the first part of the
business can best be explained by the
statement that all liquids tend to as
sumo the form of vapor, the process of
evaporation being more rapid as the
air pressure on the surface of the
liquid is lessened or its temperature
increased. This is why air takes up
portion of the water beneath 1t. Tne
capacity of air for containing the vapor
or water also varies with its tempera-
ture. At the melting point of ice
(often lnaccurately described as the
freezing point of water) air will hold in
suspension quantity of wafer equal to
one part in 100 of its own weight.
The capacity is doubled for each in
crease of 77 degrees, so that at So air
will hold one eightieth, and at sc de-
grees one 'ortieth of its weight of
water. These are the limits of what is
called "saturation.' The air above the
ocean is often near that point, while on
land it is generally below it, and over
wide arid areas the air is comp aratively
dry, but always contains some vapor of
water. This variation in capacity may
be explained by the well-known fact
that an increase of temperature drives
farther asunder the constituent air par
ticles in the atmosphere, and thus
makes more room between them for
the holding of vapor.
1t is easy to see from TINS when air
which is nearly saturated suffers a de
crease in temperature it has to part
with portion of its contained nlois,
ture, which falls either as rain, snow or
hail, according to conditions that need
not here be stated. Tiis reduction of
temperature may be caused IN one Of
several ways, chief among which may
Le ,ooa,t,0ral La fNroinn Of tha
column to a greater altitude. This is
sometimes effected by the meeting of
two air currents in blowing around the
circumference of a storm area, and
thus is accounted for the cooling due
to such meeting which has been dis
puted by some recent writers on
meteorology. The higher a given vol
ume of air is above the surface the
colder it is, ether things being equal
and the changes in elevation would
appear to be the chief causes of vari-
alton in temperature, except those due
to the long since formulated law that
the wind partakes of the character of
the surface over which it blows. The
set of the trade winds in the tropics is
well understood to be determined by
the warming of the land and water be-
meath them by the solor rays, in con
junction with the rotation of the earth
on her axis. 1t was for long time
thought that the currents in the tenn
perate zones-other than the great
one, which may be called the COM
plement OF the trade wind system
could be referred to the same origin,
but the meteorologists of to-day are
not all agreed on that point. A most
animated discussion on this subject has
recently been held, prominently par
ticipated IN by DR.. Ham IN Europe,
and Lieut.. Hazen in this country, and
they have not yet settled it. But while
they may lead up to new views OF the
causes that set air currents in motion
and thus bring about the distribution of
the moisture let down by the clouds
there is not much danger they or any
one else will dlsprove the statements
here made In regard to the process by
which water is taken up by the air and
then allowed to drop back over other
areas than those from WEIGH It was
absorbed by the thirsty air.-Chicago
I,.rA,Lno