Mammatus clouds are pouch-like protrusions hanging from the undersides of clouds, usually thunderstorm anvil clouds but other types of clouds as well. Composed primarily of ice, these cloud pouches can extend hundreds of miles in any direction, remaining visible in your sky for perhaps 10 or 15 minutes at a time.

People associate them with severe weather, and it’s true they can appear around, before or after a storm. Contrary to myth, they don’t continue extending downward to form tornados, but they are interesting in part because they’re formed by sinking air. Most clouds are formed by rising air. Mammatus clouds can appear ominous. But, in a way that’s so common in nature, their dangerous aspect goes hand in hand with a magnificent beauty.