Work out relative humidity instantly with clear inputs, formula shown and shareable results.
Relative humidity is the actual vapour pressure as a percentage of the saturation value at that temperature, computed here with the Magnus expression. Air at 22 C saturates at 26.5 hPa, so 15 hPa of actual vapour is 57% relative humidity with a dew point near 12.5 C. Warming air without adding moisture lowers relative humidity even though the water content is unchanged.
Relative humidity
RH = 100 x actual vapour pressure / saturation vapour pressure
Magnus saturation
es = 6.112 exp(17.62T / (243.12 + T)) hPa
Because heating cold outdoor air raises its saturation pressure without adding moisture, so relative humidity collapses.
Dew point is better, because it tracks absolute moisture. 20 C dew point feels humid regardless of the relative reading.