Cloud Index: Mammatus
The weirdest cloud you’ll ever see
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.
Let’s have some fun with one of the more unusual things you’ll see in the sky, shall we? Mammatus clouds are a rare phenomenon, associated with thunderstorms, where large blobs appear hanging down from the underside of a cloud. In fact they look a little like udders. Which is what gives them their name — ‘mamma-’ is the latin for ‘breasts’.
Their proper name is mammatocumulus, and are defined as a cellular pattern of pouches hanging beneath the base of a cloud. They’re seen most often underneath the anvil of a huge cumulonimbus, where they tend to indicate that a particular storm is very strong. But they can also be spotted under altocumulus, altostratus, stratocumulus and cirrus clouds, and even under ash clouds from volcanoes.
How they form
There’s quite a bit of debate over the exact mechanism that causes these pouches to form, but one thing is pretty widely agreed — it’s to do with air descending.
Clouds normally form when air rises — either due to convection (cumulus) or by being pushed up weather fronts (stratus). When air rises, it cools and the water inside condenses into droplets. But at the top of a thunderstorm conditions are a little unusual.
You’ve got a load of warm air that’s been brought up from the surface, a hard barrier above that can’t be passed (the tropopause, marking the bottom of the cloudless stratosphere), and a lot of cold air surrounding the storm. Initially the warm air spreads out below the tropopause, creating the cloud’s anvil, but as it cools down it starts to fall back towards the ground, forming mammatus in the process as the still-quite-warm air drops into a colder environment and the water vapour cools and condenses.
What they mean for the weather
Mammatus clouds tend to mean that there’s a thunderstorm around. Often they’re linked to precipitation — though that doesn’t always make it down to the surface before evaporating or sublimating.
The difficulty of getting enough warm air aloft means that a thunderstorm has to be rather strong before it can form mammatus. As such, they can be read as a harbinger of an oncoming storm, or a sign of one that has just passed. Aviators are taught to avoids cumulonimbus clouds with mammatus for this reason.
While they look impressive, they have very little impact on life at the surface. Mammatus are merely the messengers of their bigger, tougher cloud-brothers.
Anything else?
As mammatus clouds don’t directly affect humans and occur in very hazardous environments, we know relatively little about them. While they’re observed surprisingly often, it’s difficult to get scientific data back about exactly what’s going on under the thunderstorm’s anvil.
Meanwhile, their striking appearance and rarity also make them good targets for conspiracy theorists, many of whom believe that they’re the result of weather modification programmes or worse.
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