The vapour pressure of water is the pressure at which water vapour is in thermodynamic equilibrium with its condensed state. At higher pressures water would condense. The water vapour pressure is the partial pressure of water vapour in any gas mixture in equilibrium with solid or liquid water. As for other substances, water vapour pressure is a function of temperature and can be determined with the Clausius–Clapeyron relation.
Calculations of the (saturation) vapour pressure of water is most commonly used in meteorology. The temperature-vapour pressure relation inversely describes the relation between the boiling point of water and the pressure, a property used in pressure cooking.