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 Stratocumulus
clouds usually form between 1,000 and 6,500 feet (300 and 2,000
metres), whereas altocumulus clouds form between 6,500 and 23,000
feet (2,000 and 7,000 metres). Clouds at these levels are referred
to as low cloud and medium cloud respectively.
There is no great
difference in the properties of stratocumulus and altocumulus
since both are composed of water droplets and are normally
limited in vertical extent, so the distinction is rather an
arbitrary one.
Stratocumulus will
often give a sheet of almost total cloud cover, with perhaps
one or two breaks. The cloud elements are rounded and almost
join up. Occasionally, the sheet is composed of a series of
more or less parallel rolls which often, but not always, lie
'across the wind'. Also stratocumulus sometimes produces light
falls of rain or snow.
Altocumulus also provides
a sort of dappled pattern, but since it is at a greater altitude
the cloud elements look smaller. There are many variations
on the theme including altocumulus castellanus which is like
a vigorous medium-level cumulus (this type of cloud is sometimes
an indication that thunderstorms will follow).
Both stratocumulus
and alto cumulus are formed by weak convection currents, perhaps
triggered by turbulent airflows aloft. The convection affects
a shallow zone because dry, stable air above the cloud sheet
prevents further upward development.
Sometimes there are
huge sheets of stratocumulus covering thousands of square
kilometres around the flanks of a high pressure system, especially
over the oceans. The weather below such sheets tends to be
dry, but it may be rather dull if the cloud is two or three
thousand feet thick.
Useful links
Cirriform
Cumulus
Cumulonimbus
Stratus
Summary
Nature of clouds
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