Altostratus clouds are mid-level, gray or blue-gray clouds that usually cover the whole sky. If you see altostratus clouds, a storm with continuous rain or snow might be on its way.
Altostratus clouds are mid-level clouds. Although they get their names from the Latin—“altus,” meaning “high”—all clouds with the prefix “alto” are mid-level clouds.
Altostratus clouds form between 6,500 and 20,000 feet (1981–6,096 m) in the sky.
Low-level clouds–cumulus, stratus, and stratocumulus–form below 6,500 feet (1,981 m). High-level clouds–cirrus, cirrocumulus, and cirrostratus–form above 20,000 feet (6,096 m). Other mid-level clouds besides altostratus are altocumulus and nimbostratus.
The bottom of altostratus clouds can be found at about the same height as altocumulus clouds. Altostratus clouds, however, are usually much, much bigger.
Altostratus clouds form when warm, moist air rises above cold air. Though associated with warm fronts, altostratus clouds can appear all year long and they happen worldwide.
Altostratus clouds sometimes appear as thin veils that seemingly envelop the sun or moon. “Watery sun” is a term used to describe thin altostratus clouds in front of the sun. Most often, however, altostratus clouds are thick clouds that obscure the sun entirely.
Altostratus cloud layers are usually 6,500 to 10,000 feet (2,000 – 3,000 m) thick. They commonly cover miles and miles of earth.
Altostratus clouds are full of moisture. Up top, the moisture can harden into frozen ice crystals, while at the bottom the moisture is water droplets. Altostratus clouds may produce some light snow or rainfall, but they are often a precursor to heavier and darker storm clouds.