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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.
Useful links
Cirriform
Cumulus
Stratus
Stratocumulus
and Altocumulus
Summary
Nature of clouds
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