ObserveStratocumulus
Sc · Low · 24 September 2026 · By
Stratocumulus: the most common cloud on Earth
If you averaged the whole sky over the whole planet over a whole year, stratocumulus would win. It covers something like 20 to 25 per cent of the Earth's surface at any moment, mostly over the oceans, and it is the cloud that decides how much sunlight the planet reflects. It is also the cloud that most often gets called 'just grey' and then mis-identified.
Stratocumulus
Stratocumulus
- Band
- Low
- Altitude
- 600 m – 2 km
- Code
- Sc
- Seasonal window
- Marine stratocumulus peaks in summer over the eastern subtropical oceans and in winter over mid-latitude oceans. Over land it is most frequent in autumn and winter under high pressure. (Jan–Dec)
- Published
- 24 September 2026
- By
Weather clue
A closed stratocumulus deck with a steady wind means a stable marine or post-frontal airmass and a good chance of drizzle. Scattered open cells with plenty of blue mean a showery, unstable airmass with a shower risk.
How to observe it
- Sketch the deck at 09:00, 11:00 and 13:00 and shade the blue gaps; the change in gap area is the forecast.
- Measure the cloud base by timing a rising cumulus element against a known landmark if any cumulus is embedded.
- Note the roll alignment with the surface wind; parallel rolls mean shear and often mean the deck is being organised, not destroyed.
- Feel for drizzle on the back of your hand whenever the deck is closed and the wind is under 6 m/s.
The two things that make it stratocumulus
First, it is a layer: the elements are joined, or nearly joined, into a sheet. Second, it has structure: you can see individual rolls, lumps or cells, and there are usually gaps. Remove the structure and you have stratus. Remove the layer and you have cumulus.
The classic stratocumulus look is a dark, lumpy underside with lighter tops visible through the gaps, and the shading under each element is the giveaway. If there is no shading at all, look again — you may be looking at altostratus.
The varieties, and what closes the deck
The most useful distinction in the field is whether the deck is closed (no blue visible) or open (blue visible between elements), because a closed deck is much more likely to persist and to produce drizzle.
| Form | Look | Likely evolution |
|---|---|---|
| Stratocumulus stratiformis | Broad flat sheet of joined elements | Persistent; may produce drizzle |
| Stratocumulus cumulogenitus | Spreading out from the tops of cumulus | The cumulus has hit a stable layer; convection is capped |
| Stratocumulus lenticularis | Lens shapes in a low layer | Wind shear at low levels, often coastal |
| Stratocumulus undulatus | Long parallel rolls | Wind shear; expect the rolls to align with the wind |
| Stratocumulus mammatus | Pouch-like pouches hanging underneath | Recent downdraught; usually short-lived |
Why it is so persistent over the ocean
Marine stratocumulus is one of the most stable cloud regimes on the planet, and understanding why is a good exercise in cloud physics. The cloud sits on top of a well-mixed marine boundary layer about 500 to 1500 metres deep. Above it is a sharp temperature inversion that acts as a lid. Below it is a cool ocean surface that keeps supplying moisture.
The cloud top radiates heat to space and cools, which makes the air at the top denser and causes it to sink in little cells. That sinking, dry air evaporates some of the droplets, which is exactly what keeps the cloud from thickening into rain. This is why a stratocumulus deck over the ocean can sit in the same place for days, and why it usually breaks up over land where the surface is warmer.
Field skills: predicting the break-up
A stratocumulus deck over land usually breaks up during the morning, and the break-up happens from the edges inward and from the top down as the surface warms. Three practical observations help you call it.
- Gap growth. If the existing blue gaps are widening rather than new ones appearing, the deck is breaking up.
- Brightness. A deck that is getting brighter and thinner at the top is losing water; a deck getting darker and more uniform is thickening.
- Time of day. In summer, decks that survive past 11:00 local time often survive all day; decks that are still closed at 09:00 usually open by 11:00.
- Wind. A strengthening wind tends to mix the boundary layer deeper and break the deck; a dropping wind tends to let it settle.
Rules of thumb
- Stratocumulus is the most common cloud on Earth, covering roughly a fifth of the surface at any time.
- Layer plus structure is the test: no structure means stratus, no layer means cumulus.
- Over the ocean it is extremely persistent because the dry air above the inversion evaporates the cloud top.
- Over land, watch the gaps: widening gaps mean the deck is breaking up, new gaps mean it is thinning uniformly.
| The other cloud | Comparison — Stratocumulus |
|---|---|
| 高积云 · Altocumulus | Altocumulus elements pass the thumbnail test at arm's length; stratocumulus elements are hand-sized or larger and sit below 2000 m. |
| 层云 · Stratus | Stratus has no visible elements at all and no shading; any element with a shaded underside makes it stratocumulus. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
Why is stratocumulus so persistent over the sea?
Because the cloud top radiates heat to space and cools, which produces gentle sinking inside the cloud and evaporates some of the droplets. That self-limiting process stops the deck thickening into rain, and the ocean below keeps supplying moisture, so it can sit in place for days.
Why do gaps sometimes open up during the morning?
Surface heating over land warms the boundary layer from below, which deepens the mixing and eventually reaches the inversion. Once the mixed layer reaches the cloud base, the deck is being fed by drier air from below and it begins to break up from the edges.