Definition of Water vapor & phase changes

Definition of Water vaporEvaporation is the transformation of a liquid into a gas. Like any other liquid, water may evaporate gradually or violently. Its rate is fastest at temperatures near the boiling point. [See also cavitation.]

Violent evaporation (boiling) is the formation of pure water vapor at the boiling point of water. However, the white mist that emerges from a boiling kettle is not water vapor, but a mass of droplets suspended in the air. The droplets are formed when the very hot, pure water vapor meets the cold air, and cools below the boiling point. "Steam" may refer either to the droplets or to the hot vapor from which they condense.

Condensation is the formation of liquid water either from pure water vapor or water intermixed with air. In the atmosphere, condensation produces clouds, fog and rain, the latter usually only when facilitated by cloud condensation nuclei. The "dew point" of an air parcel is the temperature to which it must cool before condensation (dew) begins to form.