Humidity is the amount of water vapor in the air, measured as mass of water per unit volume or mass of air, or as a percentage of the maximum amount the air would support without condensation, or indirectly via the dew point. Saturation of the air occurs when the water vapor pressure reaches the vapor pressure of liquid water at the temperature concerned; this rises rapidly with temperature. The physiologically tolerable humidity level falls rapidly with temperature, as humidity inhibits cooling by evaporation of sweat. Humidity can be measured with a hygrometer.