Accurate, simple equation for saturated vapour pressure over water and ice

We present and assess a simple equation for saturated vapour pressure over water and ice. The equation does not rely on an explicit integration of the Clausius–Clapeyron equation, but instead uses the equality of the Gibbs functions of the vapour and the liquid or ice in equilibrium. The resulting equation is simple, physically consistent with standard thermodynamic assumptions, uses only basic physical parameters, and is at least as accurate as commonly used empirical fits. It is further shown that the finite volume of liquid water has a negligible effect on the vapour pressure. The main variation from accurate tabulated data results from the variation of vapour and liquid isobaric heat capacities. Nevertheless, it is shown that, for the purpose of accurate calculation of saturated vapour pressure, this can usually be ignored. Simple and physically intuitive expressions for saturated vapour pressure over water and over ice are presented and analysed. The results are very accurate, and moreover are consistent with other common thermodynamic approximations used in models. The results are also flexible and can be trivially adapted for different settings.