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.
Plain English
A flat, featureless gray cloud layer at middle heights that often spreads across the whole sky and dims the sun to a dull glow.
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 atmosphere, not the highest, which is a common point of confusion.
Why Pilots Care
Helps anticipate reduced visibility or precipitation that may affect flight planning, altitude choice, and alternate airport decisions.
Analogy
Altostratus can look like a thin gray blanket pulled across the sky, with the sun showing through like a bright spot behind frosted glass.
Grounding Statement
Picture a wide gray sheet of cloud above you, not low enough to touch the ground, but broad enough to make the whole sky look muted.
Intuition Check
Do not read “alto” as meaning the highest clouds in the sky. In cloud names, “alto” usually means middle-level.
Example Sentence 1
A solid altostratus deck moved in ahead of the warm front, dimming the sun to a pale disk and dropping the ceiling for the afternoon arrivals.
Example Sentence 2
As altostratus began to cover the sun during the cross-country flight, the pilot started tracking fuel reserves and nearby alternates.