Dew Point Calculator (Magnus formula)
Temperature at which air becomes saturated (100% RH). Magnus / August-Roche-Magnus equation — accurate to ±0.4 °C over normal range.
Result
- Air temperature75.0 °F
- Relative humidity60%
- Dew point60.2 °F
- Spread (T − T_d)14.8 °F
- Comfort bandSlightly humid (60-65 °F).
- FormulaAugust-Roche-Magnus: γ = (a·T)/(b+T) + ln(RH/100); T_d = (b·γ)/(a−γ); a=17.625, b=243.04 °C
Step-by-step
- Convert 75.0 °F to °C: T = 23.89 °C.
- γ = (17.625 · 23.89) / (243.04 + 23.89) + ln(60/100) = 1.0665.
- T_d = (243.04 · 1.0665) / (17.625 − 1.0665) = 15.65 °C = 60.18 °F.
How to use this calculator
- Enter air temperature and relative humidity from your weather report or a digital hygrometer.
- Read the dew point directly, plus the AMS comfort-band classification.
- Use the spread (T − T_d) to predict fog/low clouds — narrow spread = high condensation risk.
About this calculator
Dew point is the temperature at which the air becomes saturated with water vapor — cool the air below the dew point and condensation forms (dew on grass, fog, cloud formation). Unlike relative humidity, dew point is an ABSOLUTE measure of moisture: a 60 °F dew point feels the same in Texas as in Minnesota, while "60% relative humidity" feels very different in summer vs winter. Meteorologists prefer dew point precisely because of this. The August-Roche-Magnus formula approximates the Clausius-Clapeyron saturation-vapor-pressure relation with constants from Alduchov & Eskridge (1996); it is accurate to ±0.4 °C between -40 °C and +50 °C with RH 1-100%. Comfort bands (American Meteorological Society): < 50 °F = dry; 50-60 = comfortable; 60-65 = slightly humid; 65-70 = humid; 70-75 = uncomfortable; ≥ 75 = oppressive. The "spread" (T − T_d) tells aviators about ceiling and visibility — small spread (< 4 °F) signals fog or low clouds.