In meteorology, a cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in the atmosphere 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 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. With sufficient saturation in the troposphere, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface. Clouds that form at very high altitudes in the stratosphere and mesosphere do not contain sufficient moisture to generate any outfall of droplets or crystals.
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-tage according to cloud-base altitude range above Earth’s surface. Clouds with significant vertical extent occupying more than one tage are often considered a distinct group or sub-group.
One physical form shows free-convective upward growth into low or vertical cumuliform heaps. Other more layered types appear as non-convective stratiform sheets, and as limited-convective stratocumuliform rolls or ripples. Both these layered forms have low, middle, and high-tage variants with the latter two identified respectively by the prefixes alto- and cirro-. Thin cirriform wisps are found only at high altitudes of the troposphere. 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 cumulonimbiform storm clouds, 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 divided into subtypes consisting of species that are often subdivided into varieties where applicable. 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.
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.
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 metaphorically 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.
Ancient cloud studies were not made in isolation, but were observed in combination with other weather elements and even other natural sciences. In about 340 BC the Greek philosopher Aristotle wrote Meteorologica, a work which represented the sum of knowledge of the time about natural science, including weather and climate. For the first time, precipitation and the clouds from which precipitation fell were called meteors, which originate from the Greek word meteoros, meaning ‘high in the sky’. From that word came the modern term meteorology, the study of clouds and weather. Meteorologica was a work of intuitive rather than scientific study. Nevertheless, it was the first known work that attempted to treat a broad range of meteorological topics.
The book De Mundo (attributed to Pseudo-Aristotle) noted, Cloud is a vaporous mass, concentrated and producing water. Rain is produced from the compression of a closely condensed cloud, varying according to the pressure exerted on the cloud; when the pressure is slight it scatters gentle drops; when it is great it produces a more violent fall, and we call this a shower, being heavier than ordinary rain, and forming continuous masses of water falling over earth. Snow is produced by the breaking up of condensed clouds, the cleavage taking place before the change into water; it is the process of cleavage which causes its resemblance to foam and its intense whiteness, while the cause of its coldness is the congelation of the moisture in it before it is dispersed or rarefied. When snow is violent and falls heavily we call it a blizzard. Hail is produced when snow becomes densified and acquires impetus for a swifter fall from its close mass; the weight becomes greater and the fall more violent in proportion to the size of the broken fragments of cloud. Such then are the phenomena which occur as the result of moist exhalation.
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