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Cirrus (cloud classification symbol: Ci) is a genus of atmospheric cloud generally characterized by thin, wispy strands, giving the type its name from the Latin word ''cirrus'' meaning a ringlet or curling lock of hair. The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of "mares' tails". On planet Earth, cirrus generally appears white or light gray in color. It forms when water vapor undergoes deposition at altitudes above in temperate regions and above in tropical regions. It also forms from the outflow of tropical cyclones or the anvils of cumulonimbus cloud. Since cirrus clouds arrive in advance of the frontal system or tropical cyclone, it indicates that weather conditions may soon deteriorate. While it indicates the arrival of precipitation (rain), cirrus clouds only produce fall streaks (falling ice crystals that evaporate before landing on the ground). Jet stream-powered cirrus can grow long enough to stretch across continents while remaining only a few kilometers deep. When visible light interacts with the ice crystals in cirrus cloud, it produces optical phenomena such as sun dogs and haloes. Cirrus is known to raise the temperature of the air beneath the main cloud layer by an average of 10 °C (18 °F). When the individual filaments become so extensive that they are virtually indistinguishable from one another, they form a sheet of high cloud called cirrostratus. Convection at high altitudes can produce another high-based genus called cirrocumulus, a pattern of small cloud tufts that contain droplets of supercooled water. Cirrus clouds form on other planets, including Mars, Jupiter, Saturn, Uranus, and possibly Neptune. They have even been seen on Titan, one of Saturn's moons. Some of these extraterrestrial cirrus clouds are composed of ammonia or methane ice rather than water ice. The term ''cirrus'' is also used for certain interstellar clouds composed of sub-micrometer-sized dust grains. ==Description== Cirrus cloud ranges in thickness from to , with an average thickness of . There are, on average, 30 ice crystals per liter (96 ice crystals per gallon), but this ranges from one ice crystal per 10,000 liters (3.7 ice crystals per 10,000 gallons) to 10,000 ice crystals per liter (37,000 ice crystals per gallon), a difference of eight orders of magnitude. The length of each of these ice crystals is usually 0.25 millimeters long, but they range from as short as 0.01 millimeters or as long as several millimeters.〔 The ice crystals in contrails are much smaller than those in naturally-occurring cirrus cloud, as they are around 0.001 millimeters to 0.1 millimeters in length.〔 Cirrus can vary in temperature from to . The ice crystals in cirrus clouds have different shapes in addition to different sizes. Some shapes include solid columns, hollow columns, plates, rosettes, and conglomerations of the various other types. The shape of the ice crystals is determined by the air temperature, atmospheric pressure, and ice supersaturation. Cirrus in temperate regions typically have the shapes segregated by type: the columns and plates tend to be at the top of the cloud, whereas the rosettes and conglomerations tend to be near the base.〔 In the northern Arctic region, cirrus tend to be composed of only the columns, plates, and conglomerations, and these crystals tend to be at least four times larger than the minimum size. In Antarctica, cirrus are usually composed of only the columns, and these columns are much longer than normal.〔 Scientists have studied the characteristics of cirrus using several different methods. One, Light Detection and Ranging (LiDAR), gives highly accurate information on the cloud's altitude, length, and width. Balloon-carried hygrometers give information on the humidity of the cirrus cloud but are not accurate enough to measure the depth of the cloud. Radar units give information on the altitudes and thicknesses of cirrus clouds. Another data source is satellite measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) program. These satellites measure where infrared radiation is absorbed in the atmosphere, and if it is absorbed at cirrus altitudes, then it is assumed that there are cirrus clouds in that location. The United States National Aeronautics and Space Administration's (NASA) MODerate resolution Imaging Spectroradiometer (MODIS) also gives information on the cirrus cloud cover by measuring reflected infrared radiation of various specific frequencies during the day. During the night, it determines cirrus cover by detecting the Earth's infrared emissions. The cloud reflects this radiation back to the ground, thus enabling satellites to see the "shadow" it casts into space. Visual observations from aircraft or the ground provide additional information about cirrus clouds.〔 Based upon data taken from the United States using these methods, cirrus cloud cover was found to vary diurnally and seasonally. The researchers found that in the summer, at noon, the cover is the lowest, with an average of 23% of the United States' land area covered by cirrus. Around midnight, the cloud cover increases to around 28%. In winter, the cirrus cloud cover did not vary appreciably from day to night. These percentages include clear days and nights, as well as days and nights with other cloud types, as lack of cirrus cloud cover. When these clouds are present, the typical coverage ranges from 30% to 50%.〔 Based on satellite data, cirrus covers an average of 20% to 25% of the Earth's surface. In the tropical regions, this cloud covers around 70% of the region's surface area.〔 Cirrus clouds often produce hair-like filaments—similar to the virga produced in liquid–water clouds—called fall streaks, and they are made of heavier ice crystals that fall from the cloud. The sizes and shapes of fall streaks are determined by the wind shear. Cirrus comes in four distinct species; Cirrus ''castellanus'', ''fibratus'', ''spissatus'', and ''uncinus''; which are each divided into four varieties: ''intortus'', ''vertebratus'', ''radiatus'', and ''duplicatus''.〔(【引用サイトリンク】title=Cirrus – Clouds Online )〕 ''Cirrus castellanus'' is a species that has cumuliform tops caused by high-altitude convection rising up from the main cloud body. ''Cirrus fibratus'' looks striated and is the most common cirrus species. ''Cirrus uncinus'' clouds are hooked and are the form that is usually called ''mare's tails''. Of the varieties, ''Cirrus intortus'' has an extremely contorted shape, and ''cirrus radiatus'' has large, radial bands of cirrus clouds that stretch across the sky. Kelvin-Helmholtz waves are a form of cirrus intortus that has been twisted into loops by vertical wind shear. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cirrus cloud」の詳細全文を読む スポンサード リンク
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