Frost Point Calculator

The frost point is the temperature at which water vapor in the air deposits directly as ice (frost) on a surface below 0 C, without passing through the liquid phase. The dewpoint is where water vapor condenses to liquid water. Below 0 C, the saturation vapor pressure over ice is lower than over liquid water, so the frost point is slightly lower than the dewpoint for the same humidity conditions.

The frost point is calculated using the saturation vapor pressure over ice. The WMO approximation uses es_ice(T) = 6.1078 * exp(21.8745584 * T / (T + 265.49)) in hPa, where T is in Celsius. Given RH and air temperature, the actual vapor pressure is e = RH/100 * es(T), and the frost point is where this equals saturation vapor pressure over ice.

Frost damage to plants depends on both the frost point and the exposure duration. Most tender crops are damaged when surface temperatures drop to 32 F (0 C). Hardy crops tolerate lower temperatures. Agricultural frost advisories are issued when minimum temperatures are expected to reach 33-36 F with clear skies and light winds, as radiative cooling may push surface temperatures below 32 F even if the air is warmer.

Road surfaces radiate heat to the sky faster than the air above them, especially on clear nights with low humidity. The road surface can reach the frost point and drop below 32 F while the air temperature measured at 5 feet above ground is still above freezing. This is why black ice can form even when weather stations report temperatures above 32 F.

Frost forms by deposition of water vapor directly onto cold surfaces (sublimation frost) or by freezing of surface moisture. Freezing rain is supercooled liquid water that freezes on contact with surfaces below 32 F. Frost forms under calm, clear, cold conditions; freezing rain requires a warm layer aloft to melt falling snow or ice pellets, with a subfreezing surface layer.