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. Clouds in Earth’s atmosphere are studied in the cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air becoming saturated; cooling the air or adding water vapor to the air. In general, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface.
The international cloud classification system is based on the fact clouds in their most basic forms can show free-convective upward growth like cumulus, appear in non-convective layered sheets such as stratus, or take the form of thin fibrous wisps, as in the case of cirrus. Prefixes are used in connection with clouds to express variations or complexities in these basic forms or to specify middle or high altitude ranges. These include strato- for low clouds with limited convection that form mostly in uneven layers, cumulo- for complex highly-convective storm clouds, nimbo- for thick layered clouds of some complexity that can produce moderate to heavy precipitation, alto- for middle clouds, and cirro- for high clouds; the latter two of which may be of simple or moderately complex structure. Whether or not a cloud is low, middle, or high level depends on how far above the ground its base forms.