CONDENSATION
HOW HAZE, RAIN, SNOW, HAIL, FROST, CLOUD, AND FOG ARE FORMED
=Haze= is what might be called diluted cloud or fog; it differs from
them only in the degree of its density. One may see several miles
through a haze, because the minute particles of spheres of water or
ice are far apart in comparison to what they are in fog or cloud.
=Raindrops= vary in size from O.03 to O.20 of an inch in diameter.
Each drop is composed of literally millions of minute specks of water
that have condensed each about a minute mote of dust. These motes
are a million of times below anything that may be seen with the most
powerful microscope. Recall what is said in Chapter IV about the size
of the molecules in water: if a raindrop were enlarged to the size of
the earth, the molecules of which it is composed would be no larger
than a baseball, and the smallest of them no larger than tiny green
peas. Without free surfaces upon which condensation may begin there
can be no rainfall. Dust motes furnish these surfaces; without them
air may be supersaturated without condensation occurring except where
it comes in contact with solid matter. The little spherical masses
of water join together so as to form raindrops in some manner not
well understood. When enough of them coalesce so that the weight of
the drop is too heavy to be supported by the motions of the air it
falls to the ground, or is evaporated by the warmer and drier lower
air. Raindrops form mainly in the stratum between one and three miles
above the earth. It is seldom that the stratum of air next the earth
is saturated, even during rainfall. One might evaporate millions
of gallons of water and find no dust as a residue, or at least
nothing visible to the human eye, so infinitesimal are the motes of
condensation. As high as thirty millions have been shown to exist in
a single cubic centimeter of air (Chapter IV), and a million times
that number could occupy such space without being visible, and the
dust mote is composed of molecules, and the molecules of atoms. It
is impossible for the human mind to grasp the idea of the degree of
smallness to which the atom attains, and when one tries to conceive
of the electrons from which the atom is built up, he must try to
think of them not as objects but as the place or condition where
matter slowly fades away into nothing; as the place possibly where
matter is transmuted into electrical energy and ceases to exist.
The raindrop cannot be formed at great altitudes because the vaporous
atmosphere is confined to low levels by temperature. At 100°, which
often exists at the bottom of the atmosphere, air at saturation
contains 19.77 grains the cubic foot; at 80°, 10.93; at zero, .04;
and at -40°, which always may be found at about four and one half
miles high, air cannot contain in excess of .01 of a grain. Raindrops
are mainly caused by the cooling of air down to its dew point.
=Rain Water Is Not Pure.= Hailstones often incase foreign matter that
has been carried upward by violent winds. Rain water is pure when
it is condensed, but it gathers other matter as it falls, such as
the pollen of plants, and the broken siliceous shells of microscopic
life carried by winds of the tropics; it also washes ammonia from the
air in small quantities,—about thirty pounds per acre in the eastern
half of the United States each year. A raindrop increases in velocity
as it falls until the resistance of the air becomes just equal to
the weight of the drop; after that it falls at a uniform rate. It
will surprise many to learn that if it were not for the retardation
effected by the resistance of the air, a raindrop falling from only
half a mile would be as dangerous to life as a rifle bullet, for
the speed with which a projectile travels can be made sufficient to
compensate for its softness or yielding qualities.
=How Much Water Is It Possible to Precipitate from the Earth’s
Atmosphere?= If the entire amount of water vapor present in the
atmosphere were precipitated instantly it would furnish a rainfall
of only two inches for the whole surface of the earth. A steady
downpour for twenty-four hours usually amounts to some two or three
inches. Over small areas and in exceptional cases as many feet have
been known to fall in that time, as fresh, vapor-bearing winds
steadily blew into a storm center, rose, discharged their burdens as
they cooled with ascent, and then flowed away, again to be charged
with moisture when they came into contact with wet surfaces. It is
impossible to drown the entire earth with rainfall, no matter how
long continued.
[Illustration: FIG. 31.—SNOW CRYSTALS.]
=Snow.= Snow is water vapor condensed in the congealed form, without
passing through the liquid state. When the minute pieces of ice of
which the flake is composed are magnified several hundred times they
are found to be composed of the most wonderfully beautiful figures.
Thousands have been photographed, but the versatility of nature is
so great that no two ever have been found that were exactly alike.
Figure 31 gives some idea of their infinite variety and perfect
symmetry. They are always governed by the number six. The most
common form at the beginning of winter is a six-rayed star, each ray
branching. As the winter advances and the cold becomes more severe,
the flakes take a simpler form, finally becoming slender six-sided
prisms with sharp ends, under the influence of severe cold waves.
Great pain is inflicted on the exposed parts of the body when these
prisms are encountered in a high wind.
When condensation takes place in a warm stratum it will be in the
form of minute massive spherical particles or spherules. If these
spherules are then whirled aloft by ascending currents it is possible
for them to be cooled to far below the freezing point without turning
to ice; they will, however, congeal instantly when they touch one
another or are jostled by touching any solid or liquid surface. They
may give a coating of ice to the limbs of trees and the coating may
increase until the limbs break, and the surface of the earth thus
may be covered with thin ice called _sleet_.
=Hail.= There is a difference of opinion among meteorologists as
whether the thunderstorm whirls about a vertical axis, like the
tornado and the hurricane, or whether it rotates about a horizontal
axis. One may well account for the formation of the hailstone by
assuming that its alternating layers of snow and ice are caused by
the horizontal roll of a thunderstorm, the under part of which has
a temperature at or above freezing and the upper half much below
freezing. A raindrop is formed in the lower part, frozen in its
course through the upper part, receives a fresh coating of water
or snow with each revolution and a freezing before its circuit is
completed. It thus gains in size until it becomes too heavy to be
sustained by the whirling storm-cloud, when it falls to earth. Hail
usually has the size of small peas, but occasionally it falls in
chunks sufficiently large to kill cattle in the fields. On August 15,
1883, a hailstone weighing eighty pounds is said to have fallen in
Kansas.
=Frost.= Frost is composed of beautiful crystallizations, similar to
snow. Chapter VII describes the process of formation in detail.
=Cloud.= Cloud is formed by the cooling by expansion as currents of
air are carried aloft. Clouds are composed of minute watery droplets
or of ice spiculæ, depending on their temperature, and the latter
largely is determined by elevation. A cloud differs from mist or
rain in the size and number of its particles, and from fog in its
position and the method of its formation. There are three fundamental
formations, the cirrus, cumulus, and stratus. The others are
combinations of these. The cirrus are thin, high, veil-like clouds,
always composed of ice spiculæ; the cumulus look like great banks
of snow with bulging, oval tops in which thunder heads may form;
the stratus spread out like a great blanket. The cirrus usually fly
at the top of the storm stratum, some five to seven miles high; the
other clouds at some lower level. When rain is falling from a cloud,
it is called nimbus.
=Fog Is Cloud at a Low Level.= It is formed by warm water vapor
rising from lakes or rivers into the cool night air at the bottom of
valleys, or by the cold waters of oceans being forced up over a bar,
where the coldness that they impart to the adjacent air condenses
some of its vapor.
=Artificial Rain Making.= Many swindlers have preyed upon the
credulity of the public by claiming to have a process for the making
of rain, and in some cases large sums of money have been paid by
commercial or other associations to these charlatans. In 1892 the
United States Congress appropriated $20,000 for the testing of
the theory that rain could be created by the setting off of large
quantities of explosives. The experiment was unsuccessful, as the
scientists of the Government insisted it would be. The Greeks had a
popular belief that when a host of their soldiers went out to meet
an army of Persians the vapor rising from the hot breath, blood, and
sweat of the struggling mass was later condensed into rain by the
concussion of the battle clubs and the hoarse cries of the victors,
and many of the veterans of our Civil War were firm in the opinion
that their great battles were followed by rains that were the result
of the cannonading. Both the Greeks and our American soldiers were
mistaken. Rain often has fallen at the close of great battles, not
because of the concussion of the conflict, but because rain falls
on an average of one day in three in the regions where most of the
great battles have been fought, and the movement of armies began
on the fair days when travel was good. If it were the custom to
begin battles on rainy days we would have the contrary and equally
erroneous theory that concussion clears the atmosphere.
=Prevention of Hail by the Firing of Guns.= Even a Papal decree was
not entirely effective in preventing the people in southern Europe
from ringing the church bells to prevent the formation of hail when a
storm threatened, and within the past quarter-century large grants of
public money were foolishly wasted in the firing by the vineyardists
of France and other parts of Europe of a gun specially designed to
destroy hail clouds. These guns sent harmless smoke rings a few feet
aloft. The writer felt constrained to employ the extensive machinery
of the Weather Bureau to counteract the effect of glowing accounts
of the success of these guns that were sent to this country by some
of the ignorant persons employed by this Government to represent us
as consuls abroad. Even though the hail-destroying guns occasionally
were choked with hail it was difficult for scientists to prevail upon
the public to stop their foolish and wasteful practice.