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The Dew Point is the air temperature at which a sample of air would reach 100% humidity based upon its current degree of saturation.
Once the relative humidity of a mass of air becomes 100%, then if the temperature falls it cannot hold all of the water vapour within it. The excess water vapour will then condense into cloud.
When observing successive weather reports (METARs), a reducing gap between the actual temperature and the Dew Point temperature gives an indication of impending low visibility conditions and the posibility of fog.