mammatus clouds, named for their resemblance to the mammary gland, form when air laden with big water droplets is carried to the top of a thunderstorm cloud whose altitude is cold enough to freeze the water droplets. the resulting crystals sink back down towards earth, collecting at the base of the cloud before they have time to evaporate. mammatus clouds are usually only stable for a few minutes.
photos by zack schnepf and mike hollingshead in the american midwest (click pic for specific location).

















