What is the frost point?

The frost point is the temperature to which air must be cooled (at constant pressure and moisture content) for water vapor to deposit directly as ice (frost) on a surface. It differs from the dew point, which is the condensation temperature over liquid water. Below 0 °C, the frost point is always a few tenths to a few degrees warmer than the dew point.

How does the frost point differ from the dew point?

Both are computed from the Magnus formula, but with different empirical constants tuned for ice versus liquid water. Below 0 °C, saturation vapor pressure over ice is lower than over supercooled liquid water, so the frost point is slightly warmer (less negative) than the dew point. Above 0 °C the frost point is physically defined but becomes equal to the dew point.

Which formula constants does this tool use?

Dew point: a = 17.625, b = 243.04 °C (August-Roche-Magnus for liquid). Frost point: a = 22.587, b = 273.86 °C (Magnus for ice, as cited by Alduchov & Eskridge, 1996). Accuracy is within ±0.2 °C for temperatures between -60 °C and 0 °C.

When does frost actually form?

Frost forms when a surface temperature reaches or drops below the frost point, provided the air temperature is also below 0 °C. Even if air temperature is slightly above 0 °C, surfaces exposed to clear night skies can radiate heat and cool below the frost point — a phenomenon called radiative frost.

Can the frost point be above 0 °C?

Technically the calculation can yield a result above 0 °C for very high humidity, but frost deposition requires temperatures below freezing. Above 0 °C, condensation (dew) forms instead of frost, so the frost point result is only practically meaningful below 0 °C.

Why is frost point important for aviation?

Ice accretion on aircraft wings changes aerodynamic lift characteristics and can be dangerous. Ground crews use frost point data to decide whether de-icing is required before departure, especially when the surface temperature of the aircraft is at or below the frost point.

Frost Point Calculator

Enter air temperature and relative humidity to instantly calculate the frost point — the temperature at which water vapor deposits as frost on surfaces.

Enter Conditions

Temperature Unit

Air Temperature (°F)

Relative Humidity (%)

1% Dry 50% 100% Saturated

Enter temperature and humidity, then click
Calculate Frost Point.

Frost Point Temperature

-- °F

-- °C

Air Temp

Dew Point

Humidity

--%

Frost Risk Guide

No Frost Risk

Air temp above 0 °C (32 °F)

Frost Possible

Air temp 0 to -2 °C (32 to 28 °F)

Frost Likely

Air temp -2 to -5 °C (28 to 23 °F)

Severe Frost

Air temp -5 to -10 °C (23 to 14 °F)

Extreme Frost

Air temp below -10 °C (14 °F)

Frost point: Magnus ice formula (a = 22.587, b = 273.86 °C). Dew point: Magnus liquid formula (a = 17.625, b = 243.04 °C).

Summary

The Frost Point Calculator uses the Magnus formula constants calibrated for ice sublimation (a = 22.587, b = 273.86 °C) to compute the temperature at which water vapor transitions directly to ice without passing through liquid. Unlike the dew point, the frost point is only physically meaningful below 0 °C and is consistently a few degrees warmer than the dew point in subfreezing conditions. This distinction matters for accurate icing risk assessment on roads, aircraft, and outdoor structures.

How it works

Enter the current air temperature in Fahrenheit or Celsius.
Enter the relative humidity as a percentage (0–100).
The calculator first derives the dew point using the Magnus liquid formula (a = 17.625, b = 243.04 °C).
It then derives the frost point using the Magnus ice formula (a = 22.587, b = 273.86 °C).
Results are shown in both Fahrenheit and Celsius with a frost risk classification.

Use cases

Assess frost and black ice risk on roads and pavements overnight.
Plan aircraft de-icing operations before cold-weather flights.
Protect crops and plants by predicting overnight frost deposit.
Evaluate condensation and icing risk on outdoor pipes and structures.
Understand freeze-thaw cycles for civil engineering and construction.
Monitor sub-zero storage conditions in cold-chain logistics.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu