In meteorology, a cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in theatmosphere above the surface of a planetary body. These suspended particles are also known as aerosols and are studied in the cloud physics branch of meteorology.
Terrestrial cloud formation is the result of air in Earth's atmosphere becoming saturated due to either or both of two processes: cooling of the air and adding water vapor. With sufficient saturation, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface.
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 introduced in December 1802 and became the basis of a modern international system that classifies these tropospheric aerosols into several physical forms, then cross-classifies them as low-, middle- and high-étage according to cloud-base altitude range above Earth's surface. Clouds with significant vertical extent occupying more than one étage are often considered a distinct group or sub-group. One physical form shows free-convective upward growth into low or vertical heaps of cumulus (cumuliform). Other forms appear as non-convective layered sheets like low stratus (stratiform), and as limited-convective rolls or ripples as with stratocumulus(stratocumuliform). Both of these layered forms have middle- and high-étage variants identified respectively by the prefixes alto- and cirro-. Thin fibrous wisps of cirrus are a physical form found only at high altitudes of the tropopshere (cirriform). In the case of clouds with vertical extent, prefixes are used whenever necessary to express variations or complexities in their physical structures. These include cumulo- for complex highly convective vertical nimbus storm clouds (cumulonimbiform), and nimbo- for thick stratiform layers with sufficient vertical depth to produce moderate to heavy precipitation. This process of cross-classification produces ten basic genus-types or genera, most of which can be subdivided into species and varieties. Synoptic surface weather observations use code numbers to record and report any type of tropospheric cloud visible at scheduled observation times based on its height and physical appearance.
While a majority of clouds form in Earth's troposphere, there are occasions when they can be observed at much higher altitudes in the stratosphere and mesosphere. 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. These three main atmospheric layers that can produce clouds, along with the lowest part of the cloudless thermosphere, are collectively known as thehomosphere. Above this lies the heterosphere (which includes the rest of the thermosphere and the exosphere) that marks the transition to outer space. 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.
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