relative humidity

https://doi.org/10.1351/goldbook.R05267
The ratio, often expressed as a percentage, of the partial pressure of water in the atmosphere at some observed temperature, to the saturation vapour pressure of pure water at this temperature.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2210 (https://doi.org/10.1351/pac199062112167)