Cloud Index: Stratus
A thick layer of cloud that blankets the sky
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.
The tropics are all about cumulus. The Sun is warm enough to drive the convection processes that create it. But towards the poles, the ground slopes away from the Sun’s rays and there isn’t enough energy to create it. Instead, cloud formation is driven by large masses of air moving around.
This is the land of stratus, which is latin for “layer”. Stratus clouds are the ones we see on overcast days when the sky is one big featureless expanse of grey. Like cumulus, they form low in the sky — so low, in fact, that they often touch the ground, producing dense fog.
Depending on the ambient temperature they can be comprised of water droplets, supercooled water droplets or ice crystals.
How they form
Stratus clouds require large masses of air for their formation, as they cover the entire sky. When a sheet of warm, moist air rises from the ground, it cools and the amount of water the air can contain drops. When its reaches a certain point, it can’t hold the water it contains any more and it condenses into cloud.
As more moist air is fed up from below, pushed by a weather front perhaps, the cloud continues to grow vertically until it forms a thick layer that blots out the sky above. Eventually the pushing from below stops or it hits the bottom of the stratosphere and the top of the cloud stops rising.
What they mean for the weather
Stratus clouds are best-known for their association with drizzle — light, persistent rain that lasts for hours. They can also drop light snow and medium-strength rain, but you’ll rarely get a proper downpour out of one — you’ll need a cumulus cloud for that. At night, stratus clouds act as a blanket that traps air underneath it. This keeps the surface far warmer than it would otherwise be.
Stratus clouds usually form under very stable conditions, so they stick around for a while. Sometimes they can remain in place for days under anticyclonic conditions. Eventually, however, the Sun will break through the layers of cloud and it’ll shatter into stratocumulus instead.
Anything else?
If you’re a nervous flier and you see stratus clouds, that’s a good sign — they won’t cause much turbulence because the motions inside are all going in one direction. They’re not quite as good for the views though.
A thin layer of stratus appears white, whereas a thicker layer will be a darker grey as less light makes it through from above.
On other planets, stratus clouds dominate. Gas giants and smaller planets with thick atmospheres, like Venus, tend to be covered in vast layers of thick stratiform cloud.
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