Relative Humidity
The ratio of the actual vapor pressure of water in an air parcel to the saturation vapor pressure at the same temperature, expressed as a percentage.
Governing formula
RH = (e / e_s(T)) × 100%
SI unit
Percentage (%)
In depth
Because saturation vapor pressure e_s(T) increases exponentially with temperature according to the Clausius-Clapeyron equation, cooling an air parcel increases its relative humidity until it reaches 100% (dew point temperature).
Examples in the real world
Nighttime cooling raising relative humidity to 100%, causing water vapor condensation into morning dew or fog.