A heavy dark grey mass of cloud of great vertical height which usually results in heavy shower or snow is termed as

Cumulonimbus clouds are tall, dense, dark clouds associated with thunderstorms and heavy precipitation. They grow vertically through multiple atmospheric layers due to strong updrafts. Cirrus are thin, wispy high-altitude clouds; cirrostratus are thin, sheet-like clouds that create halos; nimbostratus are dark, low clouds that produce steady rain but lack cumulonimbus's vertical development.