Relative humidity is also approximately the ratio of the actual to the saturation vapor pressure.
RH = (Actual Vapor Pressure) / (Saturation Vapor Pressure) X 100%
Where actual vapor pressure is a measurement of the amount of water vapor in a volume of air and increases as the amount of water vapor increases.
Saturated vapor pressure is the maximum VP that can exist at any given temperature.
Air which is at 100% relative humidity (RH) contains water vapor whose VP is its SVP at the given temperature. This corresponds to air which is in equilibrium with liquid water. RH is the ratio VP/SVP expressed as a percentage. 'Dry' air will contain water vapor with a VP which is less than the SVP at the given temperature.
What is humidity? Humidity is the amount of water vapor in the air. As water evaporates, it changes from liquid to gas, from liquid water to water vapor. That amount of water vapor in the air is humidity. As more water evaporates and turns to water vapor, humidity increases. Warm air can “hold” more water vapor than cold air. Humidity
What is relative humidity? Relative humidity is the amount of water vapor in the air compared to the total amount of water vapor needed to reach saturation. Saturation occurs when the rates of evaporation and condensation are equal. Relative humidity is 100%. Air can become saturated when evaporation adds water vapor to the air, or when the air is cooled to its dew point. A psychrometer measures relative humidity. Relative Humidity