Altostratus is a grey or bluish, uniform sheet that covers all or part of the sky at the middle level. The sun shows through it as if behind frosted glass: a pale disc with no clear outline, casting no shadow. It is the intermediate stage between the fine veil that leads a disturbance and the thick layer that brings the rain.
- Level: Middle level (2,000 to 7,000 m)
- Base height: 2,000 to 7,000 m
- Appearance: grey or bluish sheet, sun as if behind frosted glass
- Associated weather: rain or snow close, especially if the sky lowers
How altostratus forms
Like cirrostratus, altostratus comes from a slow, general rise of moist air, typically ahead of a warm front or inside a depression. The layer thickens and sinks towards the middle level, and the ice crystals at the top mingle with water droplets. An altostratus can also come from the spreading of a large cumulonimbus whose anvil has thinned out.
In the upper part of the cloud you find mostly ice crystals; lower down, droplets, sometimes supercooled. The layer is thick enough to absorb a good share of the light, which is why the sun is dimmed. When it becomes thick enough to drop precipitation that reaches the ground, it is called nimbostratus.
How to recognise altostratus
Three signs are enough. The sky is uniformly grey or bluish, with no separate elements. The sun, or moon, is visible only as a blurred disc, not as a dazzling point. And there are no shadows on the ground and no halo, which distinguishes it from cirrostratus. A slightly thinner layer shows lighter patches, and the day looks dull.
Compared with stratus, a lower and darker cloud, altostratus is higher and a lighter grey. Compared with nimbostratus, it has no continuous precipitation reaching the ground, though light rain or snow may fall beneath it.
What weather altostratus signals
An altostratus that appears after a veil of cirrus and cirrostratus and then lowers signals rain or snow within hours. It is the surest sign of a warm front coming: the gap between the first cirrus and the rain is often several hours.
The wind generally strengthens as it approaches, the pressure falls and in winter the air turns milder. If the sheet breaks or lifts, the disturbance is passing offshore and fair weather returns.
Risks and precautions
Altostratus is not dangerous. What matters is preparing for what follows: continuous precipitation can last from several hours to half a day. In winter, snowfall is possible if the air is cold enough. For long outings, plan shelter and check the country's official warnings before you leave.
In the sky over Toronto today
Here is what the Open-Meteo forecast of 2026-11-02 08:13 gives for Toronto, averaged over the next 24 hours: 77% of the sky covered at low level, 58% at middle level and 76% at high level.
For this kind of cloud the level that matters is this one: at middle level the sky is partly covered (58% on average).
These percentages are model values on a grid: they say how much of the sky is occupied at each level, not which kind of cloud is there. They change at every update.
Frequently asked questions
How do I tell altostratus from cirrostratus?
By the sun. Behind cirrostratus it keeps a sharp outline, with a possible halo and distinct shadows. Behind altostratus it is blurred, like through frosted glass, with no halo and no shadow.
Does altostratus make it rain?
Seldom much. It can give light rain or snow, but the steady precipitation comes when it develops into nimbostratus.
Why is the sky grey and featureless?
Because the layer is thick and uniform, made of droplets and crystals that scatter light in all directions. No contrast forms, so no features show.
Sources
- World Meteorological Organization (WMO), International Cloud Atlas, WMO-No. 407: definitions of the genera, levels and base heights for temperate regions. https://cloudatlas.wmo.int (copyright WMO; no text or image from the Atlas is reproduced here).
- Drawing and text on this page: original to this site, schematic.