Question: I saw these weird clouds the other day. Couldn’t get a photo, but they looked upside down compared to most clouds. Like, instead of being flat on the bottom and puffy at the top, these were puffy at the bottom. What was that? — JL, Bristol, UK
Answer: I’m pretty sure you are describing what are commonly called “pillow clouds” because of their appearance. The technical name is mammatus clouds, since they hang down under the main cloud like a mammalian udder. We don’t get them much around here. The photo above is my best example.
Here’s a better example from Wikimedia Commons showing mammatus clouds above San Francisco Bay. Attribution to Brocken Inaglory.
There are even more extreme examples on Wikipedia. Definitely worth taking a look:
Whereas “normal” clouds are formed by rising air (hence the puffy tops), mammatus clouds form by falling air. There are several different mechanisms invoked to explain their formation. All involve the rapid cooling of air pockets in a cloud. The subsequent increase in density makes them heavier than surrounding air, so they fall to a lower altitude till they reach air of equal density.
Meteorologists don’t actually consider mammatus a “category” of cloud, like cirrus or stratus. They see the mammati (plural of mammatus) as “features” that can occur an many types of clouds. According to Wikipedia:
Mammatus are most often associated with anvil clouds and also severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altostratus, and cirrus clouds, as well as volcanic ash clouds.
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