Cumulonimbus clouds produce thunderstorms, blizzards, torrential rain, hailstorms and tornadoes. Though they have a low base, these clouds are very deep, and they can extend miles into the atmosphere.
These clouds are so tall that though they're made of water droplets in their lower levels and made of ice crystals near the top. They are so dense that they are dark and menacing when viewed from the ground. This is because light is scattered by the many water and ice droplets in the cloud as it passes through.
A cumulonimbus cloud is created when convection currents interact with unstable air. Warm, moist air rises into the cloud, which leads to condensation and more warming. This contributes to fierce updraft currents. The ice crystals at the top of the cloud fall down through the cloud to create downdraft currents. These emerge from the bottom of the cloud as strong wind, which cools the air in the updrafts. Eventually this convection system stops, and the cloud finally decays.
However, in the act of decaying, the cumulonimbus cloud can release all its moisture in an intense storm. In other instances, the updrafts and downdrafts can separate and produce a supercell. This can produce a tornado.