The Relative Humidity (RH) is the amount of water vapour in the air at any particular temperature compared with the maximum amount that it will hold at that temperature, i.e., when saturated, and is stated as a percentage, e.g., air at 10°C and 100% RH (i.e., saturated) if heated to 21°C would then be at 50% RH because air at 21°C is capable of holding twice as much moisture as air at 10°C. Conversely warm air at 21°C and 50% RH, if cooled down to 10°C, say by coming into contact with cold water pipes or window panes, would be at saturation point and any further reduction in temperature would result in condensation of water vapour on the cold surfaces. High relative humidity increases the feeling of warmth in a high temperature, and the feeling of coolness in a low temperature. Low relative humidity, or excessive dryness of the air also produces uncomfortable effects, such as dry nose and throat.

Water content of air at 50 and 100% relative humidity (RH)