Cumulonimbus cloud. Cumulonimbus are large, turbulent clouds which carry rain or hail. Cumulonimbus clouds usually form from puffy cumulus clouds at a much lower height which grow vertically instead of horizontally, giving the cumulonimbus a characteristic mushroom shape. The flat anvil-like mass of high cloud at upper left, known as an anvil dome, hammerhead or thunderhead, is a clear sign of an impending thunderstorm. It forms because the cloud has reached the tropopause, the barrier between the troposphere and the stratosphere, and cannot rise any higher into the warmer air above it. Photographed in Mongolia.