Vapor Pressure of Water Over Ice

It is the pressure of water vapor in equilibrium with a flat ice surface at a given temperature — the point where sublimation and deposition balance. Below 0 °C it is lower than the saturation pressure over supercooled liquid water at the same temperature, and the temperature at which air becomes saturated with respect to ice is called the frost point (the ice equivalent of the dew point).

Below 0 °C, water vapor can be in equilibrium with either ice or supercooled liquid water, and the two equilibrium pressures are not equal — the pressure over ice is lower. That difference is what drives the Wegener–Bergeron–Findeisen process in clouds, where ice crystals grow at the expense of supercooled droplets, and it is why frost (deposition onto ice) forms under conditions where dew would not.

Murphy & Koop (2005) is the modern reference for vapor pressure over ice, valid down to about 110 K, and this calculator uses it as the baseline. Buck (1996), the Alduchov–Eskridge Magnus form, and Goff–Gratch each have ice-specific coefficient sets that agree with Murphy–Koop to a fraction of a percent across normal conditions; the comparison table shows exactly how far each deviates at your temperature.

Both are the temperature to which air must cool to reach saturation, but the frost point is defined with respect to ice and the dew point with respect to liquid water. When the air is below 0 °C, the frost point is slightly higher than the dew point for the same moisture content. Meteorological relative humidity is conventionally reported over liquid water even below freezing (the WMO convention), so the two are reported separately.