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.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 form in various altitude ranges or étages.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-étage 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 storm clouds with significant vertical extent through more than one étage, prefixes are used whenever necessary to express variations or complexities in their physical structures. These include cumulo- for complex highly convective cumulonimbiform thunder clouds, and nimbo- for thick 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.