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In general, the physical process by which a vapor becomes a liquid or solid; the opposite of evaporation, although on the molecular scale, both processes are always occurring.

In meteorological usage, this term is applied only to the transformation from vapor to liquid; any process in which a solid forms directly from its vapor is termed deposition, and the reverse process sublimation. In meteorology, condensation is considered almost exclusively with reference to water vapor that changes to dew, fog, or cloud. Condensation in the atmosphere occurs by either of two processes: cooling of air to its dewpoint, or addition of enough water vapor to bring the mixture to the point of saturation (that is, the relative humidity is raised to 100 percent). When either of these processes occurs, condensation ensues only if condensation nuclei or other surfaces are present. In the complete absence of such, condensation does not occur at nominal saturation. The spontaneous formation of liquid or solid droplets from water vapor (homogeneous nucleation) is opposed by the surface free-energy increase that attends the creation of new surfaces of the liquid or solid phase. Only for extreme supersaturation does this free-energy balance swing in favor of spontaneous nucleation.

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