Relative humidity tells us how close we are to saturation, but doesn't directly tell us the actual amount of vapor in the air. Meteorologists prefer to know the actual vapor amount, because then they can more accurately forecast the formation of storms and clouds. The dew point temperature gives this information in a form that is useful for forecasting not only storm formation, but the minimum temperature and the occurrence of dew also.

By cooling air at constant e, the RH increases. We keeping going until the RH reaches 100% (saturation); at that point, we have cooled the air to its dew point temperature. Note that this value is a temperature, but it does not affect the actual air temperature, which is usually higher than the DP (if T = DP, we have saturated air and RH = 100%).

The greater the dew point depression (difference between T and DP), the lower the RH.

Also note that raising the actual value of e raises the dew point. This is because we are cooling air until e = e_sat to find the dew point, and e_sat increases as T increases; thus e increases as DP increases. This direct correlation between the vapor pressure e and the dew point temperature is how meteorologists know the actual vapor pressure from the dew point.