1. . 
The water content of air compared to the water content that the air could hold if it were saturated, expressed as a percentage. Air in equilibrium with fresh water is saturated with water vapor, so its RH = 100%. Air above a saturated NaCl solution has RH = 75%. Air above a saturated CaCl2 solution has RH = 31%. RH can, therefore, be used as an indicator of the water activity of a solution with which air is in equilibrium. RH can also reflect the aqueous-phase activity of an oil-emulsion mud, the basis for the Chenevert Method for testing oil muds.
Alternate Form: RH (Relative Humidity)