A cloud is a visible mass of droplets of water or frozen crystals suspended in the atmosphere above the surface of the Earth or other planetary body. Clouds in the Earth's atmosphere are studied in the nephology or cloud physics branch of meteorology.
Cloud types or genera are grouped into three general categories: cirriform (mainly detached and wispy), cumuliform or convective (mostly detached and heaped, rolled or rippled), and stratiform (mainly continuous layers in sheets). These designations distinguish a cloud's physical structure and process of formation. All weather-related cloud types form in the troposphere, the lowest major layer of the Earth's atmosphere. Cumulus-category clouds are the product of localized convective lift. If the air is only slightly unstable, clouds of limited convection will form that show both cumuliform and stratiform characteristics. With greater airmass instability caused by a steeper temperature gradient from warm or hot at surface level to cold aloft, cumuliform clouds of free convection will form and rise to greater heights. Stratus-category clouds generally form as the result of non-convective lift of relatively stable air, especially along slow moving fronts and around areas of low pressure. Most cirrus category clouds are non-convective but occasionally acquire a tufted or turreted appearance caused by small scale high altitude convection.
