Cumulonimbus cloud is a 'heaped' rain cloud. (Latin 'nimbus' translates as 'rain'). Cumulonimbus cloud can be viewed as a bigger, more impressive version of it's little brother, the cumulus cloud. Occurring worldwide, except the Antarctic, cumulonimbus are most commonly found in tropical regions. They are low level clouds that extend vertically upwards to great heights, sometimes 6 miles high or more, and their base can equally be 6 miles or so across. Most commonly associated with thunderstorms, cumulonimbus cloud produces some of the world's most violent weather.

Cumulonimbus cloud is composed of liquid droplets closer to ground level, and ice crystals high up in the upper regions. The cloud is characterised by great thermal currents with the moist air inside racing upwards at 100 mph (ten times the speed of a normal cumulus). As this warm, moist, extremely unstable air rises rapidly it cools and its moisture condenses which releases latent heat allowing the air to continue to rise. As this happens air is drawn into the cumulonimbus cloud to replace the rapidly ascending air and the cloud continues to grow.

Now ice crystal and hailstones fall through the cloud from great heights and produce downcurrents of air that exit at the base as very strong winds and start to cool the air in adjacent upcurrents. Eventually this suppresses the upcurrents and the convection ceases. As the convection stops so the cumulonimbus cloud will begin to dissipate. The life span of these clouds is, therefore, very short, just a few hours at most. However once the cloud starts to dissipate it releases its moisture - and in a big storm this can be as much as 280,000 tonnes. This of course leads to the torrential rains associated with cumulonimbus cloud.

A 'supercell' is produced when the upcurrents and downcurrents separate so that they no longer interfere with each other. These clouds may now last for several hours. And if the wind speed and direction change in the up levels then there may be sufficient rotation for tornadoes to form.

Sometimes cumulonimbus are half hidden or totally embedded within other clouds, but on other occasions, when the downdraughts are strong they can, they can be well defined in clear skies. With these conditions the classic anvil shaped cloud may be visible from many miles away.

Because these clouds have great depth cumulonimbus are very dark when viewed from beneath as light is scattered and reflected by the ice crystals and liquid droplets in the cloud. Cumulonimbus cloud can be seen at any time of day, but are more likely to be spotted in the afternoon during spring and summer when convection is at its greatest.