Description Altostratus and differences with similar clouds | Previous Top Next

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
(a) Cirrus
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.
(b) Cirrostratus
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.
(c) Altocumulus and Stratocumulus
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.
(d) Nimbostratus
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.
(e) Stratus
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.