reezes, 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