Stratocumulus is the sheet of large grey and white rolls or patches that covers the sky at the low level. It is the commonest cloud genus on Earth and the main cause of grey days without rain, especially under the high pressure of winter, when the sky can stay closed for days on end.
- Level: Low level (0 to 2,000 m)
- Base height: 0 to 2,000 m
- Appearance: large grey and white rolls or patches, joined together
- Associated weather: grey, dry sky; drizzle or light rain possible
How stratocumulus forms
Stratocumulus forms beneath a temperature inversion, a layer in which the air, instead of cooling with height, warms. This stable layer acts as a lid. Moist air, stirred by convection or wind, rises up to it, reaches saturation and fills with droplets, but the inversion stops it climbing further: cumulus then spread out into a sheet.
It also appears when a layer of stratus breaks into separate elements, or when cumulus flatten out late in the day as convection fades. Under high pressure, sinking air (subsidence) maintains the inversion and can trap this grey sky for several days: the so-called anticyclonic gloom.
How to recognise stratocumulus
Its elements are large: more than 5° of sky, wider than three fingers at arm's length. They are grey at the base and lighter on top, in rows or rolls often fused together. Gaps of blue may remain between them, and the sun breaks through at times.
It differs from altocumulus by the size of its elements and its lower level, from stratus by its relief and breaks, and from cumulus by its elements being joined along the whole layer, with a flat top.
What weather stratocumulus signals
Stratocumulus usually brings nothing more than a grey sky: no rain, or drizzle and light showers of rain or snow when the layer is thick enough. In winter, under an anticyclone, it can last several days and the sun stays hidden. In summer it often clears in the afternoon as the ground warms.
If the gaps widen, the weather is improving; if the layer thickens and lowers under a veering wind, a disturbance is approaching.
Risks and precautions
There is no direct danger. Its effects are rather psychological and practical: little light, short days, a feeling of heaviness. For solar power, it cuts output sharply. In the mountains it can hide the summits and make navigation hard; on the road, visibility stays fair as long as the layer does not drop to the ground.
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 low level the sky is mostly covered (77% 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
Does stratocumulus bring rain?
Rarely. It can produce drizzle, odd drops or light showers when the layer is thick. The sky usually stays grey and dry.
Why does the sky stay grey for days in winter?
Because an inversion under an anticyclone traps moist air near the ground. It cannot rise, so it condenses into a sheet of stratocumulus that has no reason to disperse while the situation lasts.
How do I tell stratocumulus from altocumulus?
By element size and height. Stratocumulus elements span more than 5° and are low; altocumulus elements span 1° to 5° and lie at the middle level.
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.