How it works
Relative humidity is the amount of water vapor in the air as a percentage of the most it could hold at that temperature. It equals the actual vapor pressure divided by the saturation vapor pressure.
Because warm air can hold more water, the same moisture content gives a lower relative humidity on a hot afternoon than on a cool morning. The dew point stays the same; the RH swings.
Saturation vapor pressure is found with the Magnus formula (WMO coefficients a = 17.62, b = 243.12 °C), evaluated at the air temperature and at the dew point.
Formula
RH = 100 × e(Td) ÷ e(T), e(x) = 6.112 × exp(17.62x ÷ (243.12 + x)) hPa
Questions people ask
What indoor humidity is best?
Most guidance, including from ASHRAE and the EPA, suggests about 30–60%, with 30–50% often recommended to limit mold and dust mites while avoiding dry skin and static.
Why does RH drop when I heat my house in winter?
Heating raises the temperature without adding moisture. Outdoor air at 0 °C and 80% RH warmed to 21 °C drops to about 19% RH.
What does 100% humidity mean?
The air is saturated: temperature equals the dew point, and any further cooling causes condensation as dew, fog or cloud.