national classification of clouds adopted by most
countries is a great help to both meteorological
personnel and pilots. It tends to make cloud
observations standard throughout the world, and pilots
that can identify cloud types will normally take the
necessary steps to avoid those types dangerous to their
Clouds have been divided into etages, genera,
species, and varieties. This classification is based
primarily on the process that produces the clouds.
Although clouds are continually in a process of
development and dissipation, they do have many
distinctive features that make this classification
Etages.Observations have shown that clouds
generally occur over a range of altitudes varying from
sea level to about 60,000 feet in the tropics, to about
45,000 feet in middle latitudes, and to about 25,000 feet
in Polar Regions. By convention, the part of the
atmosphere in which clouds are usually present has
been vertically divided into three etageshigh, middle,
and low. The range of levels at which clouds of certain
genera occur most frequently defines each etage.
Cirrus, cirrocumulus, and cirrostratus are always
found in the high etage. Altocumulus and altostratus are
found in the middle etage, but altostratus may often
extend into the high etage. Nimbostratus is always
found in the middle etage but may extend into the high,
and especially the low etage. Cumulus, cumulonimbus,
stratus, and stratocumulus are always associated with
the low etage, but the tops of cumulus or cumulonimbus
may extend into one or both of the two other etages.
The HIGH ETAGE extends from about 10,000 to
25,000 feet in polar regions, 16,500 to 45,000 feet in
temperate regions, and 20,000 to 60,000 feet in tropical
The MIDDLE ETAGE extends from about 6,500 to
13,000 feet in polar regions, 6,500 to 23,000 feet in
temperate regions, and 6,500 to 25,000 feet in tropical
The LOW ETAGE extends from near Earths
surface to 6,500 feet in all regions of Earth.
interpreter, and briefer, you will be viewing the state of
the sky with distinctly different objectives in mind. A
review of the various cloud types can help you to
associate past observer experiences with synoptic
conditions and trends.
High clouds. High clouds are described as follows:
Cirrus (CI). Cirrus are detached clouds of
delicate and fibrous appearance, are generally white
(cirrus are the whitest clouds in the sky), and are
without shading. They appear in the most varied forms,
such as isolated tufts, lines drawn across the sky,
branching feather-like plumes, and curved lines ending
in tufts. Since cirrus is composed of ice crystals, their
transparent character depends upon the degree of
separation of the crystals.
Before sunrise and after
sunset, cirrus may still be colored bright yellow or red.
Being high altitude clouds, they light up before lower
clouds and fade out much later. Cirrus often indicates
the direction in which a storm lies.
Cirrocumulus (CC). Cirrocumulus, commonly
called mackerel sky, looks like rippled sand or like
cirrus containing globular masses of cotton, usually
without shadows. Cirrocumulus is an indication that a
storm is probably approaching. The individual globules
of cirrocumulus are rarely larger than 1 degree as
measured by an observer on the surface of Earth at or
near sea level.
Cirrostratus (CS). Cirrostratus form a thin,
whitish veil, which does not blur the outlines of the
Sun, or the Moon but does give rise to halos. A milky
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 of
rain. In the tropics, however, cirrostratus is quite often
observed with no rain following.
Middle clouds. Middle clouds are described as
Altocumulus (AC). Altocumulus appear as a
layer (or patches) of clouds 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 usually are
arranged in groups, lines, or waves. This cloud form
differs from cirrocumulus by generally having larger
masses, by casting shadows, and by having no
connection with cirrus forms. Corona and irisation are
frequently associated with altocumulus.
Altostratus (AS). Altostratus looks like thick
cirrostratus, but without halo phenomena; altostratus
forms a fibrous veil or sheet, gray or bluish in color.
Sometimes the Sun or Moon is completely obscured.
Light rain or heavy snow may fall from an altostratus
cloud layer. Altostratus can sometimes be observed at