CLOUD-FORMATION--EVAPORATION.
Water exists in different forms without, however, undergoing any
chemical change. It is when condensed into the fluid state that we call
it "water," and then it is heavier than the atmospheric air and
therefore seeks the low places upon the earth's surface, the lowest of
which is the bed of the ocean. Wherever there is water or moisture on
the face of the globe there is a process going on at the surface called
evaporation. This process is much more rapid under the action of heat
than when it is colder. In other words, as the heat increases
evaporation increases within certain limits and bears some sort of a
ratio to it. Evaporation is not confined to water, but as our subject
has to deal with atmospheric phenomena we will speak of it only in its
relation to aqueous moisture.
The heat that is imparted to the earth's surface by the rays of the sun
is able to separate water into minute particles, which, when so
separated, form what is called vapor, which is transparent, as well as
much lighter than the air at the surface of the earth. Being lighter
than the air, it rises when disengaged and floats to the upper regions
of the atmosphere. The atmosphere will contain a certain amount of these
transparent globules of moisture in the spaces between its own
molecules. If the air is warm the molecules will be farther apart and it
will contain more moisture than when it is cold.
The process of evaporation is one of the most important in the catalogue
of nature's dynamics. Without it there would be no verdure on the hills,
no trees on the plains, no fields of waving grain, and no animal life
upon the land surface of the globe. Evaporation is nature's method of
irrigation, and the system is inaugurated on a grand scale, so that
there are but few neglected spots upon the face of the earth which
moisture, carried up from the great reservoirs of water, does not reach.
The rate of evaporation, other things being equal, depends upon the
extent of surface; therefore a smooth surface like that of the lake or
ocean will not send up as much vapor from a given area in square miles
as an equal area of land will do, when it is saturated with moisture,
for the reason that there is a much larger evaporating surface on a
square mile of land, owing to its inequalities, than upon an equal area
of smooth water. Of course, if the earth is dry there can be but little
evaporation. One of the effects of evaporation is to withdraw heat, and
so to produce cold in the substance from which the evaporation takes
place.
If we put water into a vial and drop regularly upon it some fluid that
evaporates readily it will extract the heat from the vial and the water
in it to such an extent that in a short time the water will be frozen.
In hot countries ice is manufactured on a large scale upon the principle
that we have just described. Water is put into shallow basins, excavated
in the earth, over which is placed some substance like straw that
readily radiates heat, and on the straw are placed porous bricks, that
are kept wet, thus furnishing a very large evaporating surface. In this
way the process of evaporation is carried on very rapidly and the heat
is extracted from the water to such an extent that it freezes, often
forming ice in one night over an inch in thickness, and this in the
hottest climates on the globe. Evaporation cannot go on in places where
the air is already saturated with moisture. When the air is dry
evaporation is very rapid, but as it becomes more and more filled with
moisture the evaporation is checked to the same degree. This fact
accounts for the difference of bodily comfort that we experience at
different times in the year when the temperature is the same. Sometimes
we are very uncomfortable although the temperature is not above 75
degrees Fahrenheit, more so even than we are at other times when the
temperature is ten or fifteen degrees higher. If the air is saturated
with moisture, even though the temperature is not above 70 or 75
degrees, the perspiration is not readily evaporated from the surface of
the body. If the air is dry the temperature may be much higher and we be
much more comfortable, because evaporation goes on rapidly, which keeps
the body not only dry, but cool. I remember passing through a desert in
Arizona where there was scarcely a green thing in sight in any
direction, and the temperature was said to be 140 degrees. I did not
suffer as much as I often have done in the East with the thermometer at
80 or 90 degrees, and there was very little show of sensible
perspiration; it was going on rapidly, however, but was being absorbed
by the dry air. This goes to show that temperature is not the only
factor to be considered when we are making an estimate of the good or
bad qualities of a climate.
Evaporation is carried on much more rapidly when the wind blows than at
other times, for the reason that the moisture is carried off laterally
as fast as it is formed, all resistance to its escape into the upper air
being removed. If the air is charged to saturation with moisture at a
certain temperature, it will remain so, and evaporation stops so long
as the temperature remains unchanged. If its temperature rises the
process of evaporation can start up, because the capacity of the air for
holding moisture has been increased. But if a temperature is perceptibly
lowered another phenomenon will manifest itself.
In the uncondensed state vaporized moisture is quite transparent, so
that we are able to see through it as we do through a pane of glass. If,
however, the body of air that is saturated with this invisible moisture
becomes suddenly chilled, the moisture condenses into cloud or mist.
If we watch a passing railroad train we shall notice a mass of fleecy
white mist floating away from the smokestack, assuming the billowy forms
of some of the clouds in summer. This cloud is produced by the sudden
condensation of steam, which was transparent before it came in contact
with the cold, outside air, the effect being much more pronounced in
cold than in warm weather. We may liken these floating globules of mist
to the dust of the earth which floats in the air, and it has not been
inaptly called water-dust. Anyone who has seen an atomizer used or has
stood at the foot of a great waterfall, like Niagara, has seen the fluid
so finely divided that it will float in the air, instead of falling to
the ground. What takes place is that a number of these transparent atoms
of moisture that are released in the process of evaporation coalesce
into one small drop or particle of water, and they will continue to
float in the air as mist or cloud until a sufficient number have
combined into one solid mass to render that mass heavier than the air,
when it falls in the form of rain.
If we live in a region--and there are such on the face of the
earth--where there is very little evaporation and consequently very
little moisture in the air, there is rarely ever a cloud seen nor is
there any rainfall, for the reason that there is no material existing
out of which to form clouds, and the clouds precede the rain. Hence, all
the artificial attempts to produce rain in these arid regions have been
futile. If a body of warm air, when saturated with invisible moisture,
is suddenly chilled by coming in contact with a cold wave, it is
squeezed like a sponge, so to speak, and the invisible particles become
visible because a number of them have coalesced as one particle; the
particles gather in a large mass, and we have the phenomenon of cloud
formation.
Clouds more generally form in the upper regions of the atmosphere
because it is normally colder in the higher regions. In some cases
clouds float very high in the air and in others very low. This is due to
two causes:
If we should send up a balloon containing air rarefied to a certain
extent it would continue to ascend only until it reached a point where
the outside air and that contained in the balloon are of the same
density. If we should send up this same balloon on different days with
the same rarefaction of internal air we should find that on some days it
would float higher than others, because the density of the air is
constantly fluctuating, as is indicated by the rise and fall of the
barometer. Now let us consider the balloon as a globule of moisture of a
definite weight, and this globule only one of an aggregation of globules
sufficient to form a cloud. We can readily see from what has gone before
that a cloud thus formed, having a definite density and weight, would
float higher some days than others.
Assuming again that the density of the air remains the same from day to
day, the clouds will still float high or low in the atmosphere from
another cause. Let us go back to our illustration of the balloon. If we
have a fixed condition of atmosphere, external to the balloon, and vary
the conditions internally, which means varying its weight, the balloon
will float higher or lower as the internal conditions are varied. Now
apply this principle to the moisture globules of which a cloud is formed
and we can understand why a cloud will float high or low from the two
causes that we have described. Clouds are of different color and
density, and this is due to the differences of the make-up of the
moisture globules of which the clouds are formed. If these globules are
in an advanced stage of condensation the cloud is darker and more
opaque. In earlier conditions of condensation the cloud will have a
bright look, which shows that it reflects most of the light, whereas in
the case of the dark cloud the light is largely absorbed.
There is a sort of notion prevailing that clouds come up from the
horizon, and in many cases they do, but they may form directly over our
heads. There always has to be a beginning, and that occurs wherever the
conditions are most favorable for condensation of vapor. If the earth is
wet and the sun is hot the evaporation may be very rapid as well as the
ascent of the invisible moisture, which carries with it the air, which
in turn expands the higher it rises, thus producing cold. This, taken
with the normal cold that exists in the higher regions, may be
sufficient to produce a sudden condensation of this ascending vapor,
which is all that is necessary to form a cloud.
The inquiry may arise, Why is the moisture condensed, almost always, in
the upper regions of the air, where it is rare? Because the more rare
and therefore expanded it is, the more moisture it will hold. This,
taken with the fact that cold currents are encountered high up,
sufficiently answers the question.
It is interesting to know that the processes of nature are
interdependent. It is not enough that we have the evaporation of
moisture that will ascend into the higher regions of the air and there
be condensed into cloud and possibly rain, but we must have the means
for distributing these conditions over a large area, and for this
purpose we have the phenomenon of wind. Why the winds blow can be
accounted for to a certain extent,--we might say to a large extent,--but
there yet remain many unsolved problems relating to wind and weather. Of
the phenomena of wind we will speak more fully in a future chapter.