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Description Altostratus and differences with similar clouds
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Altostratus
Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or
partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as
through ground glass. Altostratus does not show Halo phenomena.
Main differences between Altostratus and similar clouds of other genera
Sheets or layers of Altostratus may, on rare occasions, degenerate into patches which may be
confused with patches of dense Cirrus. Altostratus patches however have a greater horizontal
extent and are predominantly grey.
A high and thin layer of Altostratus may he mistaken for a veil of Cirrostratus. It is sometimes
possible to identify the doubtful cloud by remembering that Altostratus prevents objects on the
ground from casting shadows and that it may show a ground glass effect. If Halo phenomena
are present, the doubtful cloud is Cirrostratus.
Altostratus sometimes has gaps, breaches or rifts ; care should be exercised not to confuse it
with an Altocumulus or Stratocumulus sheet or layer showing the same features. Altostratus is
distinguishable from Altocumulus and Stratocumulus by its more uniform appearance.
A low, thick layer of Altostratus may he distinguished from a similar layer of Nimbostratus by
the presence in Altostratus of thinner parts through which the sun is, or could be, vaguely
revealed. Altostratus is also of a lighter grey and its under surface is usually less uniform than
that of Nimbostratus. When, on moonless nights, doubt exists regarding the choice of the
designation Altostratus or Nimbostratus, the layer is by convention called Altostratus, if no rain
or snow is falling.
Altostratus is distinguishable from Stratus, with which it may be confused, by its ground glass
effect. Furthermore, Altostratus is never white, as thin Stratus may be when observed more or
less towards the sun.