Cirrus (Ci) - the name derives from the Latin cirrus = curl of hair, tuft or wisp. Cirrus cloud is a member of the ten fundamental cloud types (or cloud genera) and are wispy white high-altitude cloud formations occurring between about 5 to 13km (16,600 to 40,000ft). In fact, they are the highest of the main cloud genera, popularly known as 'mares' tails they may even form in the upper troposphere.
Cirrus clouds, or cirri (plural), generally occur as thin featherlike white, silky patches or fine, narrow bands. Shaped by strong winds in the upper atmosphere they may be curved, hooked, fairly straight or randomly entangled. They may be appearing grey when dense and seen against the light, and yellow, orange, pink, purple and reddish when illuminated by the lowering sun, while lower clouds are already submerged in the Earth's shadow. Parallel bands of cirrus, with or without billows, are often associated with the jet stream, which is often only made visible by the so-called
jet stream cirrus.
Cirrus are composed of minute ice crystals, in regions where air temperature is lower than -20°C or -30°C. They may be caused by turbulence and wind shear, or by upper-tropospheric convection. Sometimes they are just blown out ice-crystals spreading from the top of a dying cumulonimbus or dissolving altocumulus.
Cirrus cloud frequently exhibit some halo phenomena, particularly mock suns and parts of haloes, shimmering in rainbow colours. Nowadays another method of cirrus formation is from the condensation trails of aircraft, often persisting for hours and spreading to cover large portions of the sky. Cirrus may be confused with cirrostratus, but true cirrus always occurs in relatively small patches or bands. Rounded cirriform heads or dense cirrus patches may be confused with cirrocumulus or even altocumulus. The only cloud type that develops from cirrus is cirrostratus.
Common types and varieties of cirrus:
Ci fibratus characterized by long fine striations; Ci uncinus looking like a hook or comma; Ci spissatus as dense cirrus patches; Ci vertebratus looking like ribs or fishbone and Ci radiatus, parallel bands apparently radiating from one point of the sky.
What do cirrus tell about the weather?
Cirrus are often an indication of the leading edge of a warm front at altitude, especially if they are spreading out from the west or south-west sector, thickening into a denser sheet of cirrostratus. The approaching depression is some 24 to 36 hours away. Jet stream cirrus are often indicating an depression or occlusion from the western sector, some 12 to 36 hours away. However, cirrus can also be true fair weather clouds. If they appear irregular and patchy, slowly shifting from easterly directions, often dissolving, they are indicating increasing high pressure and dry, sunny and quiet weather.