Cumulonimbus
Just as cumulus is heaped cloud, so cumulonimbus is a heaped rain cloud (nimbus means rain).
In many ways the rain-bearing variety can be considered as a bigger, better organised version of the cumulus. A cumulonimbus may be 10km across and extend 10km above the ground. This compares with a cumulus cloud which is typically a few hundred metres across and reaches a height of only a few kilometres. Instead of a ball of cotton wool, a cumulonimbus will resemble a huge cauliflower of sprouting towers and bulging turrets.
But there is one important structural difference in that the uppermost levels of the cumulonimbus have turned to ice and become fibrous in appearance, whereas cumulus clouds are composed entirely of water droplets. This icy section at the top may flatten out into an 'anvil' shape when the cloud is fully developed. When it reaches this stage the base is usually dark, with showers of rain, hail or snow falling. Ice or snow, however, will often melt before reaching the ground. Often the showers are quite heavy for short periods. Also, lightning and thunder sometimes occur.
Sometimes cumulonimbus will be 'embedded' or half hidden amongst other clouds. On other occasions they will be well separated and the 'anvil' may well be visible many miles away. Cumulonimbus clouds may be seen at any time of the day, but are most common inland during the afternoon in spring and summer. At these times convection is at its strongest and most organised.
The life cycle of a Cumulonimbus:
The life cycle of a cumulonimbus is usually completed in less than an hour. There are exceptions though. The 'Hampstead Storm' of 14th August 1975 was an example of a cumulonimbus cloud which managed to keep regenerating itself over one small area of London. About 170mm of rain fell in three hours, causing severe flooding.
