Humidity

Humidity, amount of water vapor in the air. Absolute humidity measures mass of water per unit volume of air. Saturation of the air, or dew point, occurs when the water vapor pressure reaches the vapor pressure of liquid water at the temperature concerned; this rises rapidly with temperature. Relative humidity, expressed as a percentage, is the amount of water in the air at any given time compared with the amount the air could hold at that temperature before becoming saturated. The physiologically tolerable humidity level falls rapidly as temperature rises, since humidity inhibits body cooling by impeding the evaporation of sweat.