The World Meteorological Organization recognizes ten cloud genera in its International Cloud Atlas.
Cloud genera are the broadest level in the international classification system. They are based mainly on a cloud’s shape, structure, and typical altitude. The ten genera are cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratus, stratocumulus, cumulus, and cumulonimbus.
The system developed from Luke Howard’s early nineteenth-century naming work, which introduced Latin terms such as cirrus, cumulus, and stratus. The World Meteorological Organization later standardized the terminology so observations could be compared across countries and forecasting services.
A genus is only the first classification level. Meteorologists can further identify species, varieties, supplementary features, and accessory clouds. This is why two clouds belonging to the same genus can still look noticeably different or signal different weather. Cloud genera should also not be confused with cloud layers, which describe altitude groups such as high, middle, and low clouds.