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 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:
What do cirrus tell about the weather?