Altostratus

Altostratus are clouds with gray or bluish layers, like a veil or fibrous sheet spread over a huge area; they are often associated with Altocumulus. They are composed of water droplets and ice crystals. They do not form halos, cover the sun and moon and can cause light and continuous precipitation. In some parts they may be thin enough to see the sun, such as through frosted glass.

They form in stable air masses, where in the place where they form the humidity is moderate and the temperature is relatively high. They often announce the arrival of a warm front and may be accompanied by some drizzle. Thicker Cirrostratus are sometimes mistaken for Altostratus, but Cirrostratus are sufficiently translucent to allow sunlight or moon penetration, and Altostratus do not produce the halo effect seen in Cirrostratus.

Sometimes virgins are observed in Altostratus. Altostratus layers almost always gradually end, being surrounded by thin and sparse cirrus formations, reminiscent of Cirrus spissatus. Predict or result from Cumulonimbus cells; in the first case they are regularly preceded by Cirrostratus clouds, and gradually thicken until thunderstorms previously hidden behind the horizon become visible, represented by hardly distinguishable Cumulonimbus.

When they evolve from the collapse of the middle and upper parts of cumuliform clouds, they often display mammatus resulting from downward drafts. Altocumulus formations may be present below a layer of Altostratus.

Subtypes
Altostratus undulatus

Altostratus radiatus

Altostratus duplicatus

Altostratus translucidus

Altostratus opacus

Altostratus virga

Altostratus praecipitatio