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In meteorology, a cloud is an aerosol comprising a visible mass of liquid droplets or frozen crystals made of water or various chemicals. The droplets or particles are suspended in the atmosphere above the surface of a planetary body.〔(【引用サイトリンク】url=http://www.erh.noaa.gov/box/glossary.htm )〕 Terrestrial cloud formation is the result of air in any of the lower three principal layers of Earth's atmosphere (collectively known as the homosphere) becoming saturated due to either or both of two processes: cooling of the air and adding water vapor. Clouds in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adaptation of Luke Howard's nomenclature. It was formally proposed in December 1802 and published for the first time the following year. It became the basis of a modern international system that classifies these tropospheric aerosols into several physical ''forms'' which can be found at various altitude levels or ''étages''. One physical form appears as non-convective ''stratiform'' sheets in stable air. If the airmass is slightly or partly unstable, limited-convective ''stratocumuliform'' rolls or ripples may appear. Both these layered forms have low, middle, and high-étage variants. Cloud types in the two upper étages are identified respectively by the prefixes ''alto''- and ''cirro''-. Thin or occasionally dense ''cirriform'' filaments are found only at high altitudes of the troposphere and may form in stable or partly unstable air. More generally unstable air tends to favor the formation of free-convective low or multi-level ''cumuliform'' heaps. Strong airmass instability or cyclonic lift can produce storm clouds with significant vertical extent through more than one étage. Prefixes are then used whenever necessary to express variations or complexities in their physical structures. These include ''cumulo-'' for complex highly unstable ''cumulonimbiform'' thunder clouds, and ''nimbo-'' for stable multi-étage stratiform layers with sufficient vertical depth to produce moderate to heavy precipitation. This cross-classification of forms and étages produces ten basic genus-types or ''genera'', most of which can be divided into sub-types consisting of ''species'' that are often subdivided into ''varieties'' where applicable. Clouds that form above the troposphere have common names for their main types, but are sub-classified ''alpha-numerically'' rather than with the elaborate system of Latin names given to cloud types in the troposphere. Clouds have been observed on other planets and moons within the Solar System, but, due to their different temperature characteristics, they are often composed of other substances such as methane, ammonia, and sulfuric acid as well as water. ==Etymology== The origin of the term ''cloud'' can be found in the old English ''clud'' or ''clod'', meaning a hill or a mass of rock. Around the beginning of the 13th century, it was extended as a metaphore to include rainclouds as masses of evaporated water in the sky because of the similarity in appearance between a mass of rock and a cumulus heap cloud. Over time, the metaphoric term replaced the original old English ''weolcan'' to refer to clouds in general. The science of clouds is nephology which is undertaken in the cloud physics branch of meteorology. 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「cloud」の詳細全文を読む スポンサード リンク
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