The dew point is the temperature the air should be cooled at constant pressure in order to attain a relative humidity (RH) of 100%. At this point the air cannot hold any water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation. The higher the dew point rises, the greater the amount of moisture in the air. This directly effects how "comfortable" it will feel outside. Many times, relative humidity can be misleading. Example a temperature of 30 and a dew point of 30 and have a relative humidity of 100%, then a temperature of 80 and a dew point of 60 gives a relative humidity of 50%. It would feel much "humid" on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity, this is because of the higher dew point.
