Dew point is the temperature to which air must be cooled, at constant pressure, before its water vapour begins to condense. Unlike relative humidity it is a conserved measure of how much water is actually in the air, so it barely changes through a day while the air mass stays put. This gives fire forecasters their most reliable daily rule: as air heats from dawn to mid-afternoon the dew point holds nearly steady, the saturation value climbs, and relative humidity collapses to its minimum in the hottest hours, then recovers overnight. A large temperature-minus-dew-point spread means a deep afternoon humidity minimum and poor overnight recovery, so fuels dry for more hours than they rewet. The confusion this resolves is expecting humidity forecasts and dew point forecasts to move together; a falling dew point means a genuinely drier air mass has arrived, while falling humidity may be nothing but the afternoon sun. After this Concept you can predict the shape of a day's humidity curve from a morning temperature and dew point.
Relative humidity and dew point are two distinct measures of atmospheric moisture: relative humidity is the percentage of moisture in the air relative to the maximum it could hold at its current temp…