Cirrus clouds exist in the upper troposphere, at altitudes between about 7 to 17 km. They can appear as wide sheets, wispy filaments, and also as subvisible cloud layers. Cirrus clouds are composed of non-spherical ice particles with particle sizes and number densities varying considerably with meteorological conditions and origin of the air in which they form. Because cirrus clouds cover up to 30% of the Earth they play an important role in atmospheric chemistry and climate. Cirrus clouds have a net heating effect on the Earth. This is because they only hardly affect the amount of incoming visible light from the sun, but efficiently absorb the outgoing infrared radiation from the Earth's surface, part of which they reemit back to the surface. Since cirrus clouds are very efficient in heating the surface, even small changes in cirrus cloud coverage may significantly alter the Earth's climate. Observations indicate that the mean global cirrus cloud occurrence frequency over the oceans has increased by several percent per decade. This increase in cirrus may be related to human activities such as air traffic. However, since the exact mechanisms of formation for cirrus clouds are still unknown it is difficult to predict how anthropogenic activities will change cloud abundances in the future.
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