Plain English Meaning

A flat, featureless gray cloud layer at middle heights that often spreads across the whole sky and dims the sun to a dull glow.

Full Definition

A middle-level cloud type appearing as a uniform gray or blue-gray sheet or layer that covers all or most of the sky, typically based between 6,500 and 20,000 feet AGL. It is composed of water droplets, ice crystals, or a mixture of both, and the sun or moon may be faintly visible through it as if seen through frosted glass.

Context Anchor

Seen in weather reports, forecasts, and cloud discussions when describing the type and height of cloud cover along a route.

Derivation

From Latin altus meaning 'high' and stratus meaning 'spread out' or 'layered.' The 'alto-' prefix in cloud naming signals the middle layer of the , not the highest, which is a of confusion.