Cumulonimbus clouds, also known as thunderheads, release hail and rain in conjunction with updrafts. Ice crystals ride such updrafts into the air and then fall through the clouds, collecting water droplets as they drop. Once water droplets freeze onto ice crystals, hailstones are created. Cumulonimbus clouds release rain when they have reached their moisture saturation point.
According to Universe Today, cumulonimbus clouds form rain in the same way that other clouds do. They are essentially masses of condensed water floating in the sky that eventually become so saturated with vapor that they begin to release moisture, which falls to the ground. Cumulonimbus clouds are associated with heavy precipitation and thunderstorms. These clouds tend to form below 20,000 feet and produce lightning by bumping up against each other and creating static charges through ionized water droplets. Not only do cumulonimbus clouds produce typical rain and hail, they sometimes cause flash flooding. However despite the heavy precipitation these clouds generate, such downpours tend to last for only roughly 20 minutes.
The University Corporation for Atmospheric Research reports that cumulonimbus clouds obtain their impressive energy from updrafts and downdrafts, which are essentially vertical winds capable of reaching speeds of 110 miles per hour. These same updrafts are responsible for forming hailstones within the clouds.