Cloud Index: Stratocumulus
Low, lumpy, grey sheets of cloud
Welcome to the Cloud Index — a regular feature on Looking Up where we profile a type of cloud, explaining how they form, what they’re made of and how to use them to forecast the weather. You can find the full list of clouds we’ve covered in the Cloud Index index.
Clouds don’t live forever. When conditions in the atmosphere change, which happens constantly, everything from the wispiest cirrus to the tallest cumulonimbus dies.
But sometimes, rather than dying, clouds merely change form. That’s where stratocumulus — a mixture of stratus and cumulus clouds — comes from.
Vast areas of the oceans are covered with huge sheets of stratocumulous cloud. They’re usually associated with dull weather — their bases are not as featureless as stratus cloud, but they’re less likely to rain on you than a cumulus is. In places, the Sun breaks through — yielding glorious crepuscular rays.
Usually they’re comprised of liquid water — the temperature isn’t low enough to turn them into ice crystals. In the winter, however, they can freeze and yield light snowfall. Occasionally they’re mistaken for altocumulus — the two are usually distinguished by the size of the the clouds.
How they form
As its name suggest, stratocumulus clouds are produced when the formation mechanisms of stratus and cumulus clouds are combined. They form either when weak convective currents create very shallow cloud layers below a large mass of dry stable air, or when cumulus clouds sag out overnight after the Sun’s heat is removed.
What they mean for the weather
Often, stratocumulus clouds are the first that you see in the morning. If the weather is dry, then the Sun’s warmth usually burns them off in late morning to leave a clear afternoon. If the weather is damp, however, they can become thick enough to produce some light rain.
Stratocumulus is also seen at the beginning or end of bad weather — so they can indicate worse weather to come, especially if preceded by cirrus clouds ahead of a warm front.
Anything else?
You can tell stratocumulus apart from altocumulus by stretching an arm out into the air. Altocumulus clouds will appear the size of your thumbnail, while stratocumulus clouds will be the size of your fist.
Oh, and if a layer of stratocumulus at night is very thin, you can often see the Moon through it — distorted to give the appearance of a corona. Stratocumulus can also produce other optical phenomena.
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