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%.
The relative humidity of an air-water mixture is also defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a given temperature (See what is vapor pressure). Thus the relative humidity of air is a function of both water content and temperature.