Cirrus clouds are thin wisplike strands, sometimes accompanied by patches. Their shape and arrangement lead to their common name of “mare’s tail”. These clouds can be so extensive that they are virtually identical to one another and hard to tell apart. Sometimes high altitude convection produces another form of cirrus called cirrocumulus. Many cirrus clouds produce hair-like filaments made of heavy ice crystals that precipitate from them. This precipitation often indicates the difference in the air motion (wind shear) between the upper part of the cloud and the air below it. Sometimes the top of the cloud is moving faster than the air below it. The wind direction in these clouds can vary as well.

Cirrus clouds are formed when water vapor freezes into ice crystals at altitudes above 26,000 feet. Due to the sparse moisture at thigh altitude, they tend to be quite thin. At this altitude, aircraft leave condensation trails (vapor trails) that can turn into cirrus clouds. This happens when the hot exhaust, which is mostly water, freezes and leaves a visible trail. When wind shear is absent, these trails appear as cirrus uncinus, which is an indication of high-level turbulence. The falling ice crystals evaporate before reaching the ground.

Cirrus clouds trap and reflect heat beneath them which causes a greenhouse effect. They also reflect sunlight to some extent. It is not known exactly if the purpose of cirrus clouds is to warm or cool the earth. Much of the difficulty in determining this lies in modeling the albedo effect (the extent of how much light is reflected from the sun) of clouds composed of variously sized and shaped crystals. Older models seem to underestimate the albedo effect of cirrus. Improved models may in turn improve climate predictions.

A high number of cirrus clouds may indicate an approaching frontal system or upper air disturbance. This typically signals a change in weather in the near future, and generally indicates that it will become increasingly stormy. These clouds can also be remnants of a thunderstorm. A large shield of cirrus and cirrostratus typically accompany the high altitude outflow of hurricanes or typhoons. Increase in air traffic is a possible cause of an increasing amount of cirrus clouds.

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