Mammatus clouds are the type of clouds that resemble a cellular...

Mammatus clouds are the type of clouds that resemble a cellular pattern of pouches hanging underneath the base of a cloud. They’re usually associated with crazy thunderstorms and volcanic ash clouds and are usually composed of ice, though some are almost entirely made of water. They’re so intense that they’re usually avoided by planes because of their danger risks. Pretty insane. source: http://en.wikipedia.org/wiki/Mammatus_cloud