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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.[1] These suspended particles are also known as aerosols and are studied in the cloud physics branch of meteorology. Terrestrial cloud formation is the result of air in any of the lower three principal layers of Earth's atmosphere (collectively known as the homosphere) becoming saturated due to either or both of two processes: cooling of the air and adding water vapor. With sufficient saturation in the troposphere, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface. Clouds that form at very high altitudes in the stratosphere and mesosphere do not contain sufficient moisture to generate any outfall of droplets or crystals.