Cloud

In meteorology, a cloud is an aerosol comprising a visible mass of minute liquid droplets or frozen crystals, both of which are made of water or various chemicals. The droplets or particles are suspended in the atmosphere above the surface of a planetary body. On Earth clouds are formed by the saturation of air in the homosphere (which includes the troposphere, stratosphere, and mesosphere) when air cools or gains water vapor. The science of clouds is nephology which is undertaken in the cloud physics branch of meteorology.