Rounded, smooth and ominous-looking, mammatus clouds are different from regular clouds because they form in sinking air rather than rising air. Precipitation is pulled up into the air by strong updrafts, but as it ascends, it condenses and spreads out horizontally. The air most saturated with liquid water or ice is denser and cooler than the surrounding air, so it sinks down into pouch-like pockets. Eventually, all the moisture and ice evaporates, but not before we get to see these awesome clouds. They’re associated with cumulonimbus clouds because they’re usually found on the undersides of anvil clouds, but although they often accompany severe thunderstorms, mammatus clouds just as often accompany perfectly serene weather.