Environment Planet Earth What in the World Are Mammatus Clouds? By Jaymi Heimbuch Writer California Polytechnic State University, San Luis Obispo Jaymi Heimbuch is a writer and photographer specializing in wildlife conservation. She is the author of The Ethiopian Wolf: Hope at the Edge of Extinction. our editorial process Twitter Twitter Jaymi Heimbuch Updated May 10, 2019 Don Miller/MNN Flickr Group Share Twitter Pinterest Email Planet Earth Weather Outdoors Conservation Mammatus clouds clue us in to impending storms Unusual yet beautiful, mammatus clouds, or mammatocumulus — which means “mammary cloud" or "breast cloud" — is the name for these pouch-like clouds that hang from the bottom of other clouds, usually cumulonimbus incus or “anvil” clouds. If they look ominous, that’s because they are. Since they’re usually associated with anvil clouds, which are an indicator of an impending thunderstorm, mammatus clouds are a pretty good cue to get indoors before the storm hits. But this isn’t always the case. Sometimes they appear after the storm is already over, and sometimes they are associated with other types of clouds that appear in less severe weather. But how do they form? Buyancy and convection is the key, states Wired: “In mammutus clouds, evaporation causes pockets of negative buoyancy as it cools the air inside the cloud. This makes the clouds puff downward instead of up like cumulus clouds, and they end up being like upside-down bubbles. The reason they are smooth is the thermal structure right below them. The speed at which the temperature drops with increased height, known as the 'lapse rate,' needs to be close to neutral... In other words, if you put a warm little bubble of air in a particular spot, it won’t go up or down much at all — no heat goes in or out. This is typical of the thermal structure of thunderstorms. Without these circumstances, you’d get more common ragged-looking clouds or cloudy wisps coming out.” So the next time you see these amazing and strange clouds, certainly take a moment to enjoy them, but then decide if it might be a good idea to get out of the way of an incoming thunderstorm!