Frost Risk Temperature Calculator
Estimate tonight's frost risk from today's temperature and humidity. The tool computes the dew/frost point — the temperature at which moisture condenses (or freezes) out of the air — and grades the risk as Low, Moderate or High, optionally checking it against your forecast overnight minimum.
Enter Values
How to use this calculator
- Enter the current air temperature in °C (negatives allowed).
- Enter the relative humidity as a percentage (must be above 0).
- Optionally add the forecast overnight minimum in °C to see how much margin you have above the dew/frost point, then read the risk level.
How it works
The dew/frost point uses the Magnus formula: γ = ln(RH/100) + 17.62·T / (243.12 + T), then Td = 243.12·γ / (17.62 − γ) in °C. Risk is graded High when the dew/frost point or the forecast overnight minimum is at or below 0 °C, Moderate when either is at or below 2 °C, and Low otherwise. If you supply the overnight minimum, the tool also reports the margin between it and the dew/frost point — a small or negative margin means frost is more likely.
Worked example
Worked example. At 5 °C air temperature and 80% relative humidity, γ = ln(0.8) + 17.62×5/248.12 ≈ 0.1319, so Td = 243.12×0.1319 / (17.62 − 0.1319) ≈ 1.8 °C — a Moderate frost risk. If the overnight forecast is 1 °C, the margin above the dew/frost point is only −0.8 °C.
Common mistakes
- Assuming the air must reach 0 °C for frost — radiation frost forms on clear, still nights when the screen-height air temperature is still a few degrees above zero, because ground-level surfaces cool faster.
- Entering humidity as a decimal (0.8) instead of a percentage (80).
- Ignoring wind and cloud cover — this index cannot see them, yet a breeze or cloud can prevent frost that the dew/frost point suggests.
Frequently asked questions
What is the difference between dew point and frost point?
They come from the same calculation. When the value is above 0 °C, moisture condenses as dew; when it is at or below 0 °C, it deposits directly as frost, so the same figure is called the frost point.
Can frost still form if the calculator says Low risk?
Yes. This is a guide based only on temperature and humidity. Localised cold-air pooling in valleys, clear skies and calm winds can produce ground frost even when the general risk looks low.
Related tools
- Apparent Temperature Calculator
- Humidity from Wet & Dry Bulb Calculator
- Vapour Pressure Deficit (VPD) Calculator
- Air Density Calculator
- Cloud Base Height Calculator
- Dew Point Calculator
Explore more in Weather & Environmental Monitoring.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.