Clouds are fascinating because it's easy to forget about them with all the looking at the ground we do, but sometimes you look up and realize you were just walking around underneath a temporary masterpiece.
The sky offers up a crazy assortment of cloud formations: They can look like flying saucers or ocean waves or long, fluffy worms. But one of the strangest formations out there is the mammatus cloud, which looks like a bunch of clustered pouches hanging on the underside of a larger cloud. Sometimes these pouches are pretty subtle, like bubble wrap, and other times they can hang down like the udders of a cow. In fact, the word "mammatus" comes from the Latin word "mamma," meaning "breast" or "udder."
Of course, clouds form the way they do because of the meteorological conditions they are born into, and very often they don't last long because so much is going on up in the sky all the time. Mammatus clouds are created under very tumultuous weather conditions. They often form on the underside of storm clouds, and although scientists aren't entirely sure why they form, they've got some pretty good ideas.
Unlike most clouds, which form due to air rising, mammatus clouds are a product of sinking air. They most often form in association with large cumulonimbus clouds, which have a lot going on inside them. One idea about how mammatus clouds form is that conditions underneath the storm cloud are so chaotic that little patches of dry and wet air warm at different rates and create a lumpy look at the underside of the cloud.
A more popular theory, however, is that mammatus formations have more to do with sublimation, which occurs when ice turns into water vapor, skipping the liquid form entirely. This theory maintains that ice crystals within the cloud become vapor and cool the air around them. This colder — and therefore denser — air starts to sink, producing a pouch at the bottom of the cloud.
Although mammatus clouds have the reputation of being harbingers of tornadoes and severe thunderstorms, that's not always the case. Although they are often associated with cumulonimbus clouds, which are essentially giant masses of unstable air, mammatus can occasionally form in other types of clouds like stratocumulus, altostratus and altocumulus.