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 Cumulus
clouds are often said to look like lumps of cotton wool. With
a stiff breeze, they march steadily across the sky; their speed
of movement gives a clue to their low altitude. Cumulus clouds
occasionally produce light showers of rain or snow.
Typically the base of cumulus
clouds will be about 2,000 feet (600 metres) above ground
in winter, and perhaps 4,000 feet (1,200 metres) or more on
a summer afternoon. Individual clouds are often short-lived,
often lasting only about 15 minutes. They tend to form as
the ground heats up during the day and becomes less frequent
as the sun's heat wanes towards evening.
The cause of small cumulus clouds
is usually convection. Heat from the sun warms the ground
which in turn warms the air above. If a 'parcel' of warm air
is less dense than the cooler air around it or above it, the
'parcel' of air starts to rise - this is known as a 'thermal'.
As it rises it expands and cools, and if cooled sufficiently
the water vapour condenses out as tiny cloud droplets. A cumulus
cloud is born.
The air within the cloud will
continue to rise until it ceases to be buoyant. On some sunny
days there is insufficient moisture or instability for moisture
to form.
In hilly regions, a high, south-facing
slope acts as a good source of thermals, and therefore of
cumulus. Occasionally, a power station, factory or stubble
fire will produce a cloud of its own.
When air rises in thermals there
must be compensating downdraughts nearby. These create the
clear areas between cumulus clouds, and make it easier for
glider pilots to find the thermals which they can use to gain
height.
Useful links
Cirriform
Cumulonimbus
Stratus
Stratocumulus
and Altocumulus
Summary
Nature of clouds
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