Cirrus (CI) clouds are detached clouds of delicate and stringy
appearance, generally white in color, without shading. They appear in
the most varied forms, such as isolated tufts, lines drawn across the sky,
branching featherlike plumes, and curved lines ending in tufts.
Cirrus clouds are composed of ice crystals; hence their transparent
character depends upon the degree of separation of the crystals. Before
sunrise and after sunset, cirrus clouds may still be colored bright yellow
or red. Being high-altitude clouds, they light up before lower clouds
and fade out much later. Cirrus clouds often indicate the direction in
which a storm lies.
Cirrocumulus (CC) clouds, commonly called mackerel sky, look like
rippled sand or like cirrus clouds containing globular masses of cotton,
usually without shadows. Cirrocumulus clouds indicate that a storm
probably is approaching.
Cirrostratus (CS) clouds are a thin whitish veil that does not blur the
outlines of the Sun or Moon but gives rise to halos (colored or whitish
rings and arcs around the Sun or Moon; the colored arcs appear reddish
on the inside edges). A milky veil of fog (thin stratus) and altostratus
are distinguished from a veil of cirrostratus of similar appearance by the
halo phenomenon, which the Sun or Moon nearly always produces in a
layer of cirrostratus. The appearance of cirrostratus is a good indication
Altocumulus (AC) clouds are a layer (or patches) composed of flattened
globular masses, the smallest elements of the regularly arranged layer
being fairly small and thin, with or without shading. The balls or
patches are usually arranged in groups, lines, or waves. Sometimes a
corona (similar to a halo but with the reddish color on the outside edges)
may be seen on the altocumulus. This cloud form differs from the
cirrocumulus by generally having larger masses, by casting shadows, and
by having no connection with the cirrus forms.
Altostratus (AS) looks like a thick cirrostratus, but without the halo
phenomena, the altostratus is a fibrous veil or sheet, gray or bluish in
color. Sometimes the Sun or Moon is obscured completely. At other
times they can be vaguely seen, as through ground glass. Light rain or
heavy rain may fall from a cloud layer that is definitely altostratus.