Clouds form when the atmosphere becomes saturated and moisture condenses out around tiny particles of dust, salt or ice, collectively referred to as condensation nuclei. The shape of the cloud reflects the turbulence of the atmosphere and signals what is happening with the weather.
Mammatus (or mammatocumulus) clouds are puffy and rounded, with a distinctive protuberance on their undersides. Their name reflects their appearance, coming from the Latin word for breasts, while ‘cumulus’ is the Latin for pile or heap. Their formation is not fully understood, but it is thought that they are the result of sinking air, usually after a storm.
If a bad storm is brewing, clouds often pile up high; the top of the pile drifts in the strong winds of the upper atmosphere so the pile becomes shaped like an anvil. This kind of cloud is called cumulonimbus and it can warn of torrential rain or snow, hail, thunderstorms or even tornadoes to follow. Mammatus clouds often form the underside of cumulonimbus clouds and so are associated with storms.