PRECIPITATION OF MOISTURE. 103

 

 

by sailors cats’ tails or mares’ tails. From their
elevation, the moisture is, probably, generally in
the condition of ice-particles. Halos, or circular

»bands of light around the sun, are caused by light
passing through cirrus clouds.

The Cumulus, or Heap Cloud, is a denser cloud
than the cirrus, and is formed in the lower re-
gions of the air, where the quantity of vapor is
greater. Cumulus: clouds generally consist of
rounded masses, in the shape of irregular heaps,
with moderately flat bases. They are caused by
ascending currents of air, which have their moist-
ure condensed by the cold produced by expan-
sion and elevation. Cumulus clouds occur dur-
ing the hottest part of the day. Their height
seldom exceeds two miles.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig, 91. Primary Forms of Clouds,

~ Nimbus. + ~ Stratus.

The Nimbus, or Storm Cloud, is any cloud from
which rain falls. Any of the various forms of
clouds may collect and form a nimbus cloud.
The nimbus is not considered as a distinct form
of cloud by some meteorologists.

The Stratus, or Layer Clouds, form in long,
horizontal sheets or bands. These clouds are
most common in the early morning and evening,
when the ascending currents are weak. They
are caused by the gradual settling of cumulus
and other clouds. The stratus is the lowest form
of cloud; it sometimes falls to the surface of the
earth, and becomes a fog.

The cirrus, stratus, and cumulus clouds assume
a variety of shapes, producing various secondary
forms.

270. Secondary Forms of Clouds.—The cirro-
stratus, the cirro-cumulus, and the cumulo-stratus

 
 

are the most prominent secondary forms of
clouds. ‘The first two are modifications of the
cirrus cloud; the latter, of the cumulus.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    

Fig, 92, Secondary Forms of Clouds.
~ Cirro-Cumulus. ~ +-Cirro-Stratus. ~~~ Cumulo-Stratus.

The Cirro-Cumulus is a cirrus cloud, arranged
in little rounded masses, shaped something like
cumuli. They are sometimes called “ wool sacks,”
and indicate dry weather.

The Cirro-Stratus is a cirrus cloud which has
settled in bands or layers. The bands are not
continuous, but are arranged in blotches or bars,
and often give to the sky the speckled appear-
ance of a mackerel’s back, producing the so-called
mackerel sky.

The appearance of a mackerel sky indicates—1. That
the moisture of the upper strata of air is condensing; 2.
That it is growing dense enough to arrange itself in

layers. Therefore, a mackerel sky generally indicates
approaching rain.

The Cumulo-Stratus is the form produced by

_ the heaping together of a mountain-like mass of

cumulus clouds; the. base partakes of the nature
of the stratus cloud, but the top clearly resembles
cumuli. These clouds differ but little from the
nimbus, or storm cloud.

271. Rain—When, during the formation of a
cloud, the condensation of moisture continues, the
drops of which the cloud is composed increase
in size, and, uniting, fall to the earth as rain.
Rain which freezes while falling forms sleet. As