Mammatus clouds are some of the most visually striking and unusual cloud formations in the sky.

They appear as rounded, pouchlike lobes that hang down from the base of a cloud, usually under cumulonimbus clouds. Their name comes from the Latin word "mamma," meaning udder or breast, which is pretty spot on when you consider their shape.

While they’re often associated with an unstable environment, mammatus clouds themselves aren’t dangerous.

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They form when cold air sinks into warmer air below a cloud, the opposite of what typically happens in rising storm clouds. This sinking air causes pockets to form in the cloud base, creating the sagging, pouchlike structures we see.

Think of these clouds as more of a visual signature of atmospheric turbulence, especially in the upper levels of the storm. Their presence often tells meteorologists that strong updrafts and downdrafts were present in the storm’s lifecycle.

And for skywatchers and photographers, mammatus clouds are a rare treat. They are extremely photogenic, dramatic, and always worth capturing.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.