Altocumulus is the cloud of the woolly sky: rounded patches, rolls or fleeces in white and grey, grouped in banks or sheets at the middle level. It is one of the commonest clouds, appearing in fair weather as well as before a summer thunderstorm, and its shape tells you whether the weather is about to turn.
- Level: Middle level (2,000 to 7,000 m)
- Base height: 2,000 to 7,000 m
- Appearance: white and grey rounded patches or rolls, in banks
- Associated weather: often fair; castellanus: thunderstorm risk
How altocumulus forms
Altocumulus forms in a fairly thin layer of moist air between about 2,000 and 7,000 m. The layer is stirred by a ripple in the air or by small convection cells. Where the air rises it cools, reaches saturation and fills with droplets; where it sinks it warms and dries, leaving a gap of blue sky between elements.
At the middle level the cloud is mostly water droplets, sometimes supercooled, with a few ice crystals. It can arise from the spreading of cumulus or from the breaking-up of an altostratus. When it is produced by waves in the air, for instance downwind of high ground, it takes the form of smooth lenses called altocumulus lenticularis: these lenses stay almost still even though the wind through them is strong.
How to recognise altocumulus
The elements of altocumulus span 1° to 5° of sky: roughly the width of one finger up to three fingers at arm's length. They almost always have a grey part, on the side away from the sun, which sets them apart from cirrocumulus, smaller and all white, and from stratocumulus, whose elements exceed 5° and lie lower.
When the sun passes behind a layer of altocumulus it sometimes lights the cloud edges in lovely colours, called iridescence. If the elements rise into little turrets on a common base, you have altocumulus castellanus, a detail that matters for forecasting.
What weather altocumulus signals
When the patches are regular, move slowly and the sky brightens between them, the weather stays settled. If, on the other hand, they thicken, fuse into a sheet and come with cirrus or cirrostratus from the west, deterioration is under way. An altocumulus sky on the morning of a hot summer day is one to watch.
Altocumulus castellanus, with their small turrets, betray instability at the middle level: the air there is ready to rise fast. When they show up early on a hot, humid morning, thunderstorms often develop in the afternoon or evening.
Risks and precautions
Altocumulus gives no significant rain: occasionally trails of precipitation hang from it but evaporate before reaching the ground. The only risk is what it announces. A sky of altocumulus castellanus on a very hot day is a reason to postpone a mountain walk or a trip out to sea, because storms can break quickly and over narrow areas.
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 high is altocumulus?
At the middle level: between 2,000 and 7,000 m in temperate latitudes, according to the International Cloud Atlas. It is higher than stratocumulus and lower than cirrocumulus.
What does "castellanus" mean?
It is a species of altocumulus whose elements rise into small turrets on a common base, like the battlements of a castle. It reveals instability in the air at mid-height, often the prelude to thunderstorms.
Does altocumulus mean rain?
Not always. It signals a change if it thickens and comes with cirrus from the west. If it thins out, fair weather holds.
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.