Cirrus clouds have a variety of forms and sizes, depending on the amount of wind
shear, whether there is any convection and whether there is any precipitation falling from the
clouds.They are high altitude clouds, usually over 6000 metres, entirely made out of ice crystals
and have a fibrous appearance where their edges are visible. They are also almost always
transparent, i.e. the disc of the sun can be seen through them, or translucent.
There are several species and varieties of cirrus cloud, but it should be noted that they to tend to grade into each other.
Cirrus fibratus over Wolstanton, August 2007
These clouds occur where the winds at high altitude (generally over 6000 metres) are strong. They can have a mare's tail appearance, but lack tufts or hooks at either end.
In Britain, cirrus clouds often mark the leading edge of a warm front, where warmer air is advancing at a very gentle angle, over cold air below. Cirrus fibratus is a particularly common form of the cloud.
Cirrus uncinus at Wolstanton, September 2007
These clouds are very similar to Cirrus fibratus, but hey have a tuft or hook at their upper end. In the area of the tuft, there is actually some limited uplift which produces a cloud dense enough to cause precipitation of fine ice crystals. As these fall they are caught up by the wind and blown away from the cloud.
Because the air at these high altitudes is usually very dry, the ice crystals suffer a process called sublimation, which involves them being transformed into gas, water vapour. The process is the equivalent of evaporation, which convert liquid into gas.
Each cloud lies within a 'column' of rising air. The clear air between the clouds is descending.
Cirrus vertebratus over Wolstanton
An unusual form of cirrus in which narrowing undulating extensions of a linear cloud create a form comparable to the backbone of a fish, hence vertebratus.
The undulating pattern of the extensions suggests wave action running parallel to the main line of the cloud, the clearer gaps corresponding to descending currents of air, and the extensions to areas of uplift.
These clouds can form at any time of year and their occurrence and lineation seem to match up with the Jet Stream.
Cirrus intortus over South Stack, Anglesey
Another rather unusual form of cirrus in which there appears to be no particular pattern at all. The cloud appears are a tangled capricious mass of fibres showing little overall alignment
Cirrostratus and Cumulus humilis at Aberffraw, Anglesey, Wales, July 2002
The highest of the stratiform clouds is cirrostratus. It occurs as thin layers at heights of over 5km, and is formed from ice crystals. During the day it can give the sky a milky white appearance. The cloud is not thick enough to obscure the sun or the moon, both of which can produce halos. Cirrostratus is often the first indication of an approaching warm front. In the top left of this photograph there is also a clearer and greyer layer of lower level stratus cloud.
Cirrocumulus at Wolstanton, August 2004
This is a thin white cloud sheet perforated by regular holes. It occurs where there is a shallow layer of convection high in the troposphere. The individual units of cloud are only the width of a little finger held at arms length. This technique can be used to separate cirrocumulus from altocumulus, the units of which are about the width of three fingers held at arms legnth. Cirrocumulus is most common in the summer months, often in the warm sector of a depression close the an approaching cold front. It regularly foretells unsettled showery or even thundery weather on the way.