deposition of a liquid or a solid from its vapour, generally upon a surface that is cooler than the adjacent gas. A substance condenses when the pressure exerted by its vapour exceeds the vapour pressure of the liquid or solid phase of the substance at the temperature of the surface where condensation (condensation nucleus) occurs. Heat is released when a vapour condenses. Unless this heat is removed, the surface temperature will increase until it is equal to that of the surrounding vapour.

If air were free of tiny particles, called aerosols, condensation would only occur when the air was extremely supersaturated with water vapour. In the atmosphere, however, there is an abundant supply of aerosols, which serve as nuclei, called condensation nuclei, on which water vapour may condense. Some are hygroscopic (moisture-attracting), and condensation begins on them when the relative humidity is less than 100 percent, but other nuclei require some supersaturation before condensation begins.

In the atmosphere the relative humidity of the air is increased, and condensation results when air temperature is reduced to the dew point or when sufficient water vapour is added to saturate the air. Condensation accounts for the formation of dew, fog, and clouds. For rain to occur, other physical processes are required.