Relative humidity (RH) expresses how much water vapour the air contains as a percentage of the maximum it could hold at that temperature. At 100 % RH the air is saturated and condensation begins — forming fog, dew, or clouds. The key concept is that warm air can hold far more moisture than cold air: at 30 °C, air can carry roughly four times more vapour than at 10 °C.
This temperature dependence is why RH varies dramatically through the day even when the actual moisture content (absolute humidity) stays constant. On a typical summer day, RH may drop to 25–30 % in the afternoon heat and climb to 90–100 % at dawn as temperatures fall to the dew point. This daily cycle explains morning dew and fog that disappear once the sun warms the air. The instrument used to measure RH is the hygrometer.
Human comfort is strongly influenced by relative humidity. Below 30 % the air feels dry, causing skin irritation and static electricity. Above 70 %, the body's ability to cool through sweat evaporation is reduced, amplifying the sensation of heat — this is captured by the heat index (or apparent temperature). For weather forecasting, high RH at low levels combined with instability signals the potential for thunderstorms, while very low RH increases wildfire risk. Indoor comfort zones typically fall between 40 % and 60 %.