CLOUD FORMATION--CONTINUED.
As water in its condensed state is 815 times heavier than air, the
question naturally comes to one why it does not immediately fall to the
earth when it condenses. There are at least two and probably more stages
of condensation. Investigators into the phenomenon of cloud formation
claim to have ascertained that the first effect of condensation is to
form little globes of moisture that are hollow, like a bubble, with very
thin walls. Everyone has recognized the ease with which a soap bubble
will float in the air, and yet it is simply a film of moisture. These
little balloons, so to speak, are called spherules. It is undoubtedly
the case that mingled with these little bubbles of moisture there are
fine particles of solid water hanging on and carried along with them.
Undoubtedly this is true; at least just before the final act of
condensation takes place; and when the little hollow spherules collapse
they are gathered together in drops of water larger or smaller according
to the rapidity of condensation. There is probably another power at
work to prevent the too ready precipitation of moisture when condensed,
and that is the wind. A cloud never stands still, although in some cases
it may appear to do so. If we take a stone in our hand and allow it to
drop without applying any force to it, it will fall directly to the
ground. But if we give it an impetus in a horizontal direction it will
travel some distance before striking the ground. If we could give the
same impetus to a body as light as a globule of water-dust it would
probably travel indefinitely without falling. Dust that would settle
directly to the ground from an elevation in still air would travel
thousands of miles without falling, before a wind having any
considerable velocity.
Suppose the sun to be shining with intense heat upon a certain area of
the earth's surface and the conditions to be right for very rapid
evaporation of moisture. The air which is heated close to the ground,
being expanded, will rise, together with the invisible particles of
moisture, and there will be a column of moisture-laden air continually
ascending until it reaches a point in the upper atmosphere where it is
condensed into a cloud that takes on the billowy form which in summer
time we call a thunder cloud, but which in the science of meteorology is
called cumulus, or heap-cloud. If there were no air currents this
billowy cloud would stand as the capping of an invisible pillar of
ascending vapor, but as it is never the case that air is not moving at
some velocity in the upper regions, it floats away as rapidly as it is
formed. This peculiar kind of cloud is formed in the mid-regions of the
atmosphere, and it is a summer cloud as well as a land cloud. Of course,
it may float off over the ocean and maintain its peculiar shape for a
certain distance, but it is rare that such a cloud would ever be seen in
mid-ocean or in midwinter. As the warm season advances in summer, and
evaporation from the earth is less than the rainfall, there is less and
less moisture in the air, when, of course, the conditions for cloud
formation, especially inland, are not so favorable as in the early
spring or summer. Frequently there comes a time when we have a long
season of dry, settled weather. Probably during most of the days clouds
will form and we think it is going to rain, but before night they have
vanished, and the same thing is repeated the next day and the next,
perhaps for weeks at a time.
The explanation is this: We have already said that so long as the air
remains in a uniform condition as to temperature it will absorb moisture
in a transparent state until it is filled to the measure of its capacity
at a given temperature. If there were no change of temperature, it would
not condense into cloud. Clouds may be absorbed into the atmosphere--or
evaporated--and become invisible; and this process is going on to a
greater or less degree continually. If we watch the steam as it escapes
from a steam boiler, the first effect is condensation into cloud, but as
it floats away it gradually melts and is absorbed into the atmosphere as
invisible vapor. This is especially true on a warm day; the same process
takes place in the air that is going on at the level of a body of water
or at the surface of moist earth.
As before stated, condensation always takes place when a body of
moisture-laden air comes in contact with cold. When the steam escapes
from a boiler, even on the hottest day, it is hotter than the
surrounding air; the first effect is condensation, and then evaporation
takes place the same as it would at the surface of the earth when the
condensed particles of moisture are separated into the invisible atoms
that accompany evaporation.
In settled, dry weather as the sun approaches the zenith, the earth
becomes intensely heated, and there is an ascending column of air partly
laden with moisture; but not to the same extent as earlier in the
season. Condensation takes place and clouds are formed, but as there is
not sufficient moisture to carry them to the point of a further
condensation,--which would result in precipitation,--as the sun lowers
in the west and the heated air becomes more evenly distributed this
condensed vapor is reabsorbed into the air as invisible moisture by a
process allied to that of evaporation. This condition of things would
extend to a much longer period than it does in our latitude if it were
not for the gradual changing of the seasons, which finally destroys the
balance in the dynamics of cloud-land and allows the cold--that has been
held back for the time--in the great northern zone to get the upper
hand. Then we have what is termed in common parlance a change in the
weather, or, more properly in this case, a change in the season.
We have already spoken of the cloud called cumulus (which means heap)
and of its performance during the dry season of summer. There is another
form of cloud that is seen at this season of the year called cirrus (a
curl). It takes the form of a curl at its ends. This cloud usually has a
threaded shape and sometimes takes the form of a feather, and frequently
forms are seen that remind you of frost pictures on a window pane. These
clouds float very high in the atmosphere, away above the tops of the
highest mountains, from six to eight miles above the level of the sea.
They are formed only at a season of the year when the atmospheric
conditions are most uniform. At certain times of the day and night the
moisture will rise to this height before it condenses and when it does
condense it immediately freezes, which makes it take on these peculiar
forms that would no doubt conform very closely to the frost pictures on
the window pane if it were not for the disturbing influences of air
currents at this altitude. The fact that they are ice or frost clouds
instead of water clouds gives them that peculiar whiteness and
brightness of appearance. If ordinary clouds are water-dust these high
clouds may be called ice-dust. Sometimes we see them lying in bands or
threads running across the sky in the direction that the wind blows.
Their form is undoubtedly a resultant of the struggle between the air
currents and the tendency of crystallized water to arrange itself in
certain definite lines or forms. This cloud may be said to be one
extreme, having its home in the highest regions of cloud-land, while the
cumulus, or thunder cloud, is the other extreme and occupies the lower
or mid regions of the air.
There is a still lower cloud of course, as ordinary fog is nothing more
than cloud, which under certain conditions lies on the surface of the
ocean or dry land. Fogs prevail when the barometer is low. As soon as it
rises from the source of evaporation the moisture condenses almost to
the point of precipitation. There is not enough buoyancy in its
globules when the air is light, as it is when we have a low barometer,
to cause the fog to float into the higher regions of the atmosphere.
The high clouds, which are called cirrus, under certain conditions drop
down to where they begin to melt into ordinary moisture globules, and
while this process is going on we have a combined cloud effect which is
called cirro-stratus. This form of cloud may be recognized, when looking
off toward the horizon, by its being formed into long straight bands. It
is sometimes called thread-cloud. As it further descends it takes on a
different form called the cirro-cumulus, or curl-heap. This is just the
opposite in its appearance to the cirro-stratus, as it is broken up into
flocks of little clouds separated from each other and in the act of
changing to the form of the cumulus, or billowy form of cloud; and this
latter takes place when it drops to a still lower stratum of warmer air
and is there called the cumulo-stratus, which is the form of cloud we
most often see in the season of thunderstorms. The lower edge of the
cloud is straight, parallel with the horizon, while the upper part is
made up of great billowy masses, having high lights upon their well
defined projections and blending into darker shades caused by the
shadows in the valleys between the mountains of cloud.
The rain cloud is called the nimbus, and may be said to be the extension
of a cumulo-stratus. When it reaches this condition it is condensed to a
point where the vesicular globules collapse and a number of them run
together, forming a solid drop of water, and here it begins to fall. It
may be very small at first, but in its fall other condensed globules
will adhere to it and if the conditions are right, sometimes the rain
drops will have the diameter of a quarter of an inch by the time they
reach the earth.
Under other conditions, such as we have sometimes during dry weather,
the rain drops will start to fall, but instead of growing larger, they
grow smaller by absorption into the thirsty air, and will not be allowed
to reach the earth. Often there are showers of rain in the air that fall
to a certain distance and are taken up, as in the process of
evaporation, to again be formed into cloud, without ever having touched
the earth.
Thus it will be seen that clouds assume various forms under various
conditions of atmosphere, as it is related to moisture, temperature, and
density. Clouds sometimes appear to be stationary when they are only
continually forming on one side and continually being absorbed into
invisible moisture on the other. I remember seeing some wonderfully
beautiful cloud effects in the regions of the Alps. Almost every day in
summer there appears above the peak of Mount Blanc a beautifully formed
cloud cap standing some distance above it and hollowed out underneath
like an inverted cup. Although this cloud appears to be stationary, it
is undergoing a rapid change; the moisture rises from the snow-capped
peak as invisible vapor to a certain distance, where it is condensed
into a cloud of wonderful brilliancy. As the cloud globules float upward
they are absorbed into the atmosphere again, as invisible moisture at
the upper limit of the cloud. If the wind happens to be blowing, another
phenomenon takes place, giving the appearance somewhat of a volcano. It
is blown off from the peak in the direction of the wind, but within a
short distance it strikes a warmer stratum of air, where it is absorbed
and assumes the transparent condition.
If we ascend a high mountain, we get some idea of the altitude of the
various forms of cloud. A thunderstorm may be in progress far below us,
while the sun may be shining from a clear sky above, with perhaps the
exception of the frost clouds that we have referred to floating high
above the mountain tops.
We have now described in a general way how clouds are formed, how they
are condensed into rain, and how moisture is distributed over large
areas by these rain clouds being borne on the wings of the wind; and now
you ask, Whence the wind? In our next and following chapters we will try
to answer this question.