ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment
ashrae.org/ashraeterms

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relative humidity (rh, RH)

(1) ratio of the mole fraction of water vapor to the mole fraction of water vapor saturated at the same temperature and barometric pressure. (2) ratio of the partial pressure or density of water vapor to the saturation pressure or density, respectively, at the same dry-bulb temperature and barometric pressure of the ambient air.