About this tool
The Frost Point Calculator finds the temperature at which water vapor in the air begins to deposit directly as ice (frost) rather than condensing as liquid dew. Below freezing, the saturation vapor pressure over ice is lower than over supercooled water, so the frost point is slightly higher than the dew point for the same air — a distinction that matters in aviation icing, refrigeration, and cold-climate agriculture.
The calculation is two steps. First the actual vapor pressure over water is found from the Magnus equation: e = 6.1094·exp(17.625·T/(T+243.04))·RH/100 in hPa. Then that vapor pressure is inverted against the saturation curve over ice using the WMO ice coefficients a = 22.587, b = 273.86°C, and es0 = 6.1121 hPa: with γ = ln(e/es0), the frost point is Tf = b·γ/(a − γ) in °C. The tool also shows the ordinary dew point for comparison.
Everything runs in your browser locally. Enter the temperature (°C or °F) and relative humidity; results appear in °C and °F. The frost point is physically meaningful below 0°C — above freezing it effectively coincides with the dew point, since frost does not form. Relative humidity must be above 0 and at most 100%.