Before revision: Relative humidity The relative humidity ( or ) of an air-water mixture is defined as the ratio of the partial pressure of water vapor () in air to the saturation vapor pressure () of water at the same temperature, usually expressed as a percentage:Perry, R. H. and Green, D. W, Perry's Chemical Engineers' Handbook (8th Edition), McGraw-Hill, ISBN 0-07-142294-3 , pp.
After revision: Relative humidity relative humidity is the ratio of  , pp.