n (1820) (RH) The ratio, always expressed in percent, of the quantity of water vapor present in the ambient atmosphere to the quantity that would saturate it at the prevailing temperature. It is also the ratio of the partial pressure of water vapor present to the vapor pressure of water at the prevailing temperature. High relative humidity, which occurs in summer in many manufacturing-plant atmospheres, can cause condensation of water on chilled surfaces such as injection molds, with attendant mold defects. It can also cause blushing problems in the painting of plastics and metals. See also Relative.