Relative humidity

Relative humidity is the amount of water vapour in the air compared with the maximum amount the air could hold at the same temperature. It is expressed as a percentage, and it rises when the air cools or when more water vapour is added. High relative humidity means cloud, mist, fog, dew or icing can form more easily when the temperature approaches the dew point. Pilots use it together with temperature and dew point to judge visibility, cloud base and the likelihood of condensation or icing.