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A cirrus cloud is a type of cloud composed of ice crystals and characterized by thin, wisplike strands, often accompanied by tufts. Sometimes these wispy clouds are so extensive that they are virtually indistinguishable from one another, forming a veil or sheet called "cirrostratus". Sometimes convection at high altitudes produces another form of cirrus called "cirrocumulus" a pattern of small cloud tufts. The name "cirrus" is derived from a Latin word meaning "wisp of hair."

Many cirrus clouds produce hairlike filaments made of the heavier ice crystals that precipitate from them. These "fall streaks", a form of virga, often indicate the difference in the motion of air (wind shear) between the upper part of the cirrus cloud and the air below it. Sometimes the top of the cirrus cloud is moving rapidly above a slower layer of air, or the streak is falling into a faster moving lower layer. The directions of these winds can also vary.

Cirrus usually form at altitudes above 5000 meters (16,000 feet). The fall streaks may appear straight when wind shear is absent, giving the clouds the appearance of a comma (cirrus uncinus), or tangled, an indication of high-level turbulence. The falling ice crystals evaporate before reaching the ground.

Cirrus clouds contribute both to the atmosphere's greenhouse effect and to the earth's albedo (the amount of sunlight the earth reflects); consequently it has not been determined for certain whether the net effect of cirrus clouds is to warm or cool the earth. Much of the difficulty lies in modelling the albedo effect of clouds composed of various size and shape crystals. Older models tended to underestimate the albedo effect of cirrus. Refinements of these models will improve climate predictions.

If there are many cirrus clouds in the sky it may be a sign that a frontal system or upper air disturbance is approaching. Cirrus clouds can also be the remnants of a thunderstorm. A large shield of cirrus and cirrostratus typically accompanies the high altitude outflow of hurricanes/typhoons. Cirrus clouds have been observed to develop after the persistent formation of condensation trails from aircraft. The effects of condensation trails on cirrus cloud enhancement is an increasingly prominent part of the discussion about aviation and climate change.

Cirrus and Aviation Cirrus (- Smog, see: Contrail) filter the sunlight, thus altering the quality and diminishing the quantity of the Sun's radiation that reaches the surface of Earth.