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 if looking through ground glass.
Altostratus prevents objects on the ground from casting shadows, and it does not show halo phenomena.
Altostratus is sometimes a result of thickening and lowering Cirrostratus.
Due to its uniform appearance, Altostratus is not subdivided into species. It does have several varieties though. Two of note are:
Altostratus translucidus – thin Altostratus, the greater part semi-transparent to reveal the position of the sun or moon; and
Altostratus opacus – thick Altostratus, the greater part sufficiently opaque to mask the sun or moon completely.
Distinguishing As from other genera
Altostratus differs from Cirrostratus in that Altostratus prevents objects on the ground from casting distinct shadows. The sun may appear vague as through ground glass. If halo phenomenon is observed, the cloud is Cirrostratus.
Thick Altostratus (opacus) is distinguished from Nimbostratus by the presence of occasional thinner parts through which the sun’s position is vaguely revealed. It is also lighter grey in colour. At night, when it is difficult to distinguish between Altostratus and Nimbostratus, it is called Altostratus if no precipitation is falling.
Altostratus differs from Altocumulus and Stratocumulus in that even if it shows gaps, breaches or rifts, it can be distinguished by its more uniform appearance, and lack of rounded masses, rolls, etc.
The precipitation associated with Altostratus is (non-showery) rain or snow or ice pellets, often of an intermittent nature.