Frost Point Calculator

What is the frost point?

The frost point is the temperature to which air must be cooled — at constant pressure and water vapour content — for frost (ice crystals) to begin forming on solid surfaces. It is similar to the dew point, but applies specifically when the resulting temperature is below 0°C (32°F).

When the dew point is above 0°C, liquid dew forms first. When the dew point is below 0°C, frost forms directly from water vapour without passing through liquid water — this process is called deposition.

The formula

This calculator first computes the dew point using the simplified Magnus formula:

Dew Point (°C) ≈ T − ((100 − RH) / 5)

Where T is air temperature in °C and RH is relative humidity in percent. When the dew point is below 0°C, it also approximates the frost point temperature, which is about 1–2°C higher than the dew point at cold temperatures due to the different vapour pressure curve of ice vs. liquid water.

Frost Point ≈ Dew Point × 1.003 − 0.37 (valid below 0°C)

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 can damage or kill frost-sensitive crops — including tomatoes, peppers, cucumbers, and most tender annuals — within minutes of exposure. 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.