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. On Earth clouds are formed by the saturation of air in the homosphere when air cools or gains water vapor.
Cloud types 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.
Uploaded: Feb 15, 2016