An Etymological Dictionary of Astronomy and Astrophysics

The process whereby the amount of → water vapor in the air exceeds that needed to → saturate. In other words, the condition of air in which the → humidity is above the level required for saturation at a given temperature (i.e. the → relative humidity is greater than 100%). When the temperature drops below freezing, this can lead to a situation where more water vapor is present in the air than the air can hold. At every temperature, there is a maximum amount of water vapor that can be supported in the air. The higher the temperature, the more water vapor can be accommodated. But if the air that is already at 100% relative humidity is cooled then it becomes supersaturated, and this situation is unstable. As a result, the excess water vapor crystallizes out, either into water droplets or directly into ice.

See also: → super-; → saturation.