Cirrus clouds generally refer to atmospheric clouds that are characterized by thin, wisplike strands, often accompanied by tufts, leading to their common (non-standard) name of mare's tail. Sometimes these clouds are so extensive that they are virtually indistinguishable from one another, forming a sheet of cirrus called cirrostratus. Sometimes convection at high altitudes produces another form of cirrus called cirrocumulus, a pattern of small cloud tufts which include droplets of freezed water. The term is also used for certain interstellar clouds composed of sub-micrometre sized dust grains.
Many cirrus clouds produce hair like 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 clouds are formed when water vapor freezes into ice crystals at altitudes above 8000 meters (26,000 ft). Due to the sparse moisture at a high altitude, they tend to be very thin. At this altitude, aircraft leave condensation trails that can turn into cirrus clouds. This happens when hot exhaust, mostly water, freezes, leaving a visible trail. Streaks may appear straight when wind shear is absent, giving the clouds the appearance of a comma (cirrus uncinus), or tangle, an indication of high-level turbulence. The falling ice crystals evaporate before reaching the ground.
Cirrus clouds cover up to 30% of the Earth and have a net heating effect. Cirrus clouds efficiently absorb outgoing infrared radiation (heat) beneath them (greenhouse effect), while only marginally reflecting incoming sunlight (albedo).
A high number of cirrus clouds may be a sign of an approaching frontal system or upper air disturbance. This usually signals a change in weather in the near future, usually becoming increasingly stormy. Cirrus clouds can also be the remnants of a thunderstorm. A large shield of cirrus and cirrostratus typically accompany the high altitude outflow of hurricanes or typhoons. Cirrus clouds have also been observed developing after the persistent formation of contrails from an aircraft. Increase in air traffic is a possible cause of an increasing amount of cirrus clouds.
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