What is Humidity?
Easily stated humidity is simply the amount of water vapor held in the air. Water vapor is the gaseous state of water. As the temperature of the air increases more water vapor can be held since the movement of molecules at higher temperatures prevents condensation from occurring.
Relative humidity, expressed as a percent, is a measure of the amount of water vapor that air is holding compared the the amount it can hold at a specific temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a higher relative humidity. A relative humidity of 50% means the air holds on that day (specific temperature) holds 50% of water needed for the air to be saturated. Saturated air has a relative humidity of 100%.
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.