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-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 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 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 (see genus classification chart, far right), 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.