Relative Humidity

RH is strongly dependent on temperature. For instance, a parcel of air at 15°C and 50% RH is cooled down to 10°C. Now, the relative humidity of this parcel of air will be close to 70%, without the actual amount of water having increased.  Relative Humidity expresses how close the air is to being saturated with water vapour. Warm air can hold more moisture in absolute terms, cold air less. If the air becomes saturated (RH 100%) water vapour will condense as mist in the air or as water on adjacent cold surfaces. 

Relative humidity is a suitable measure when the risk of condensation on surfaces or mould growth is to be evaluated.