Frost Point Calculator

What is the frost point?

The frost point is the temperature air has to be cooled to, at constant pressure and water vapour content, before frost starts forming on solid surfaces. It is the dew point’s cold-weather counterpart, and it only means anything below 0°C (32°F).

Above 0°C the vapour condenses as liquid dew. Below it, water vapour turns straight to ice without ever being liquid. That process has a name: deposition.

The formula

Both the dew point and the frost point come from the same quantity: how much water vapour is actually in the air. The Magnus-Tetens relation gives it as

γ = ln(RH / 100) + (17.625 × T) / (243.04 + T)

with T in °C and RH in percent. Saturation over liquid water then gives the dew point,

Dew Point = 243.04 × γ / (17.625 − γ)

and saturation over ice, which needs a different pair of constants, gives the frost point:

Frost Point = 273.86 × γ / (22.587 − γ)

The frost point is always the warmer of the two below freezing, and that is the whole reason it is worth calculating separately. Ice has a lower saturation vapour pressure than supercooled water at the same temperature, so air reaches saturation with respect to ice before it reaches saturation with respect to water. The gap grows as it gets colder: about 0.2°C at a dew point of −2°C, 0.6°C at −5°C, and a little over 2°C at −20°C.

You will see the rule of thumb Dew Point ≈ T − (100 − RH) / 5 quoted a lot. It is fine above about 50% humidity and near room temperature, and it falls apart outside that. At 20°C and 30% humidity it is off by four degrees, which on a frost page is the difference between a warning and no warning. This calculator does not use it.

Will frost form?

Frost can form on surfaces whenever the surface temperature drops to or below the frost point. Clear, calm nights are most conducive to frost because there is no cloud cover to retain heat and no wind to mix the air layers.

Agricultural importance

A frost event kills frost-sensitive crops in minutes: tomatoes, peppers, cucumbers and most tender annuals. Light frost (0°C to −2°C) damages tender plants. Moderate frost (−2°C to −5°C) harms most vegetables. Severe frost (below −5°C) damages even cold-hardy crops.

Gardeners and farmers should check the frost point before leaving plants unprotected on cold, clear nights, especially in spring and autumn when ground temperatures can fall much faster than the official air temperature.