Dew point

The dew point is the temperature to which air must be cooled for it to become saturated with water vapour. When air temperature and dew point are close together, relative humidity is high and cloud, mist, fog or dew can form easily. A large temperature dew point spread usually means drier air. Pilots use dew point to assess fog risk, cloud base and the likelihood of carburettor icing or airframe icing.