What is Relative Humidity?

Relative humidity is the percentage ratio of the actual water vapour contained in a given sample of air to the maximum quantity of water vapour that the sample can hold at that temperature.

A fourth term most frequently used to describe the water vapour content of the atmosphere is relative humidity (RH). This is defined as the ratio of the mass of water vapour present to that which could be present if the air was saturated at the same temperature. The ratio is expressed as a percentage.
The RH value of unsaturated air is always less than 100%. For B at 20°C, the ratio is 10: 15 giving an RH value of 66.7% (10115 x loo), and for C, with the ratio of 1.75:2.00, the RH is 87.5%. Itshould be noted that although B contains more water vapour compared with C its RH is lower, thus showing that the temperature of the sample, rather than the mass of water vapour, is the controlling
As the temperature of the sample increases or decreases, so the RH value will change. If the water vapour content and pressure are constant and the temperature of B is increased to 30°C (B,), the RH willdecrease to 37% (10127 x 100). If the temperature is decreased to 17°C (B,), the RH will increase to 90.9% (10/11 x 100).
The RH value for saturated air (A) is always loo%, and for supersaturated air (D), which may exist in the atmosphere under certain conditions, it is always greater than 100%.
Changes in relative humidity are not achieved solely by the decrease or increase of air temperature.
Saturation can result from an increase in water vapour content (B-J), or by a simultaneous increase of water vapour content and decrease of air temperature (B-K).