In meteorology, a cloud is an aerosol consisting of visible droplets or frozen crystals, both of which are made of water or various chemicals. Droplets or particles are suspended in the atmosphere above the surface of the planet. On earth, clouds are formed by the saturation of the atmosphere in the same layer (including the troposphere, stratosphere and mesosphere). Air can be cooled to its dew point by various atmospheric processes, or water (usually in the form of water vapor) can be obtained from adjacent sources. Nephrology is a branch of cloud physics in meteorology.

Cloud types in the troposphere, the atmosphere closest to the earth's surface, have Latin names because of the general applicability of the Luke Howard nomenclature. It was formally proposed in December 1802 and first published the following year. It has become the basis of modern international system, which divides these tropospheric aerosols into five physical forms and three altitudes or amplitudes. These physical types are arranged in the ascending order of convective activities, including lamellae, doughnut like bunches and patches, layered silty layers (mainly composed of rolled, wavy and massive structures), silty heaps and clusters, and very large asymmetric heaps of powders which usually show complex structures. The physical forms are classified according to altitude to produce ten basic genera or genera. Some of these basic types are common to more than one form or more than one marker, as shown in the bevel and bevel columns of the following classification tables. Most genera can be divided into several categories, some of which are common to more than one genus. These can be subdivided into varieties, some of which are common to multiple genera or species.

The main types of cirrus clouds formed in the stratosphere and upper middle layers have a common name, but they are classified by alphanumeric, rather than using a well-designed Latin name system for cloud types in the troposphere. They are relatively uncommon and are found mostly in the polar regions of the earth. Clouds have also been observed in the atmosphere of planets and satellites in the solar system and beyond. However, because of their different temperature characteristics, they are usually composed of other substances, such as methane, ammonia and sulfuric acid, as well as water.