Rain Cloud Applique

A cloud is a collection of very fine water droplets (fog) or ice crystals in the atmosphere. Clouds are visible because light is scattered due to Mie scattering, which causes the Tyndall effect to occur and the droplets, which are actually colorless, become visible.

They form, drift, and dissipate in the currents of the terrestrial atmosphere with their often varying strength and direction at different altitudes (wind shear). The water droplets form around condensation nuclei when the relative humidity of the air slightly exceeds 100% (by no more than 1%). This can occur either when the air cools as it rises (thermals, sliding up other layers of air, on a mountainside) or when two air masses mix (Richard Mollier).

During condensation, the enthalpy of condensation of water is released, which attenuates the cooling as the air continues to rise. This allows the air to rise to higher altitudes. If the air is still and there are few condensation nuclei, the air may become supersaturated with water vapor. Although the relative water content is then significantly more than 100%, condensation does not yet occur. The water content must first increase further before it condenses. At air temperatures below -10 °C, ice crystals (tiny snowflakes) can form on the condensation nuclei by resublimation. Condensation nuclei are electrostatically charged and range in size from 1 nm to 1000 nm. They are formed by fungal spores, pollen of wind-blooming plants, fine ash particles from forest and steppe fires, which are common all over the world, salt crystals of sea spray, private households, industry, car exhausts, agriculture and cosmic radiation (example cloud chamber). After condensation begins, more and more water vapor condenses at this point until it becomes visible fog droplets. In meteorological systematics, clouds are classified as hydrometeors.