A cloud is a detectable mass of water droplets or frozen ice crystals poised in the Earth’s mood 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. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Generally, precipitation will fall to the surface; an exception is virga which evaporates before reaching the surface. Clouds can show convective development like cumulus, be in the form of layered sheets such as stratus, or appear in thin fibrous wisps as with cirrus.