Mammatus clouds are formed when cold upper air sinks downward, forming pouches under the cloud layer. They tend to precede severe electrical and hail storms, and are an ominous warning of impending tornadoes.
The “Breast Cloud”: Mammatus clouds are a rare formation that often accompany thunderstorms. They are named after… well, mamma,which means “udder” or “breast”. The reasons behind their formation are cloudy, but they only appear during rapid changes of moisture, temperature, and wind speed.