The frost-point reference
Murphy and Koop (2005) reviewed the experimental vapor-pressure data for ice and supercooled water and produced equations now treated as the modern reference. The ice equation shown here is valid above 110 K and is what this site uses as the baseline for saturation over ice — the frost point.
Below 0 °C, water vapor can saturate over ice or over supercooled liquid water, and the two pressures differ. That gap is exactly why frost forms and why the frost point is distinct from the dew point.
When to use it
Use Murphy–Koop for any sub-zero or frost-point calculation. Switch the main calculator to its 'over ice' mode to compute frost points with this equation, and to compare the simpler Buck and Magnus ice forms against it.
For relative humidity reported over liquid water below freezing (the WMO convention), the dew point calculator uses the supercooled-water relationship instead.
Compare with other formulas
See this and every other formula side by side, with the live deviation from IAPWS-95 at your temperature, on the main calculator. The Antoine equation has its own page.
References
Every formula on this page is implemented from, and validated against, the following primary standards and papers.
- Review of the vapour pressures of ice and supercooled water — Murphy & Koop 2005, QJRMS 131:1539
- IAPWS R6-95(2018) / Wagner & Pruss 2002 — International Association for the Properties of Water and Steam — the reference standard
Spotted an error? Let us know — reader corrections are the best review this site gets.