Mammatus cloud, also known as mammatocumulus, is a meteorological term applied to a cellular pattern of pouches underneath the base of a cloud. Mammatus was derived from the Latin word mamma [meaning 'breast'], and refers to the resemblance between the characteristic shape of these clouds to a woman's breast.

While they may appear foreboding they are merely the messengers- appearing around, before or even after severe weather.

Typically composed primarily of ice, they can extend for hundreds of miles in each direction and individual formations can remain visibly static for ten to fifteen minutes at a time.

mammatus cloud #1 

 mammatus cloud #2