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 Cirriform
clouds (ie.clouds from the cirrus family) are found at high
altitude, usually above 20,000 feet (6,000 metres). They are
composed of ice crystals. Three types of cloud make up the group:
cirrus, cirrostratus and cirrocumulus.
Cirrus itself is very
common in the British Isles and throughout most of the world.
It is thin, wispy and white in appearance, and its name, coming
from the Latin work for 'tuft of hair' gives a good description
of the cloud. Another name for the cloud, 'mares tails', also
conjures up an accurate image. Cirrus may be hooked or straight
depending on the airflow aloft. Sometimes it comes as a very
dense patch which is left over from the 'anvil' cloud of a
cumulonimbus which has disappeared. On other occasions, cirrus
may be quite extensive when associated with a jet stream -
the cloud can then be seen moving across the sky despite its
great altitude. Aircraft condensation trails are a form of
man-made cirrus. They can sometimes be seen in 'historical'
films, to the delight of film buffs who enjoy spotting technical
inaccuracies.
Cirrostratus is a
fairly uniform sheet f thin cloud through which the sun or
moon can be seen. Sometimes, if the cloud is thin, a bright
ring of light (called a halo) surrounds the sun or moon. A
layer or cirrostratus is often an indication of a deterioration
in the weather.
Cirrocumulus is often
present in small amounts along with cirrus, but rarely does
it dominate the sky. On those occasions when it is widespread
a beautiful spectacle is created, especially at sunset. The
individual clouds appear very small - often tiny rows of roughly
spherical pear-like cloud elements. Sometimes they occur in
undulating patterns like tiny ripples.
Useful links
Cumulus
Cumulonimbus
Stratus
Stratocumulus
and Altocumulus
Summary
Nature of clouds
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