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.
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 fibratus, August 2007, Wolstanton, Staffordshire
© Paul Swinhoe
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 uncinus, 4th July 2016, Harriseahead, Staffordshire
© Lionel Burch
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 vertebratus, 8th December 2009, Wolstanton, Staffordshire
© Paul Swinhoe
Of course, this is not the official name of this cloud! It is still Cirrus vertebratus but on this occasion the main spine from which the fibres spread outwards is clearly not straight.
This is quite unusual, but on this day at Tittesworth, 6th August 2018, virtually every variant of cirrus was on display at some point of the afternoon!
Cirrus vertebratus, 6th August 2018, Tittesworth, Staffordshire
© Paul Swinhoe
This is another, rather unusual, form of cirrus in which there appears to be no particular pattern at all.
These clouds here occurred on a hot summer's afternoon and appeared as a tangled capricious mass of fibres showing little overall alignment.
Cirrus intortus,6th August 2018, Tittesworth, Staffordshire
© Paul Swinhoe
This is the name given to parallel bands of cirrus clouds that appear, due to perspective, to radiate out from a distant point.
Cirrus radiatus, June 2011, Harriseahead, Staffordshire
© Lionel Burch
This photograph was taken a short while later, as the evening encroached. It shows a distinct contrail (condensation trail) of an aircraft making a turn to the left at altitude. Contrails occur where the exhaust of a jet engine causes condensation of the air nearby. They are a sign of relatively high humidity at altitude, where warm damper air is advancing over cooler drier air, a situation typical of the leading edge of warm fronts.
Cirrus radiatus with Contrails, June 2011, Harriseahead, Staffordshire
© Lionel Burch
Very occasionally, cirrus clouds are grey and thick enough to obscure the sun or the moon. When this happens they are classified as Cirrus spissatus.
In this case, late afternoon on 6th August 2018, at Tittesworth in North Staffordshire, the clouds were near the leading edge of a frontal system
More often Cirrus spissatus is associated with the remnants of the anvil heads of dying cumulonimbus clouds, typically towards the end of the warmest part of the day, when surface heating is in decline.
Cirrus spissatus, 6th August 2018, Tittesworth, Staffordshire
© Paul Swinhoe