With no water vapour present, the partial pressure of the air must equal the barometric pressure. When water vapour is added it exerts a certain pressure regardless of whether or not the air is present. The saturated condition exists when the actual vapour pressure is equal to the vapour pressure of the pure liquid at the same temperature.
Partially saturated air contains vapour that is superheated, that is the temperature of the mixture and therefore that of the vapour is higher than the saturation temperature for the existing vapour pressure.
The relative humidity (rh) of an air-water vapour mixture is defined as the ratio of the vapour pressure existing compared to the vapour pressure at saturation for the same dry-bulb temperature. This is also equal to the ratio of the mole fractions under the same conditions.
Rh is usually express as a percentage but occasionally as decimal (less than unity).
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