The dew point temperature is a better measure of how much moisture is in the atmosphere than the commonly known relative humidity. Dew point is the point at which dew forms if the air temperature lowers to that temperature. It truly represents the amount of moisture in the atmosphere at any given time. R/H is temperature dependent. Meaning it is relative to what the temperature is at the time of the reading.
For example if you start out the morning with a temperature of say 52 degrees, and the dew point is at 49 degrees. The humidity at the time would be very high, say 94%. Now that same day the air temp. goes to 75 degrees, while the dew point stays right around 50 degrees. The R/H for these readings would be 70%. Now it would be considered not nearly as humid, but the actual moisture in the atmosphere hasn’t changed.
Dew point is much more constant, and truly represents the amount of humidity that is in the air.
Here is a scale that one can use to show how humid it is based on TD or Dewpoint.
Here is the link to Wikipedia for Dew Point. http://en.wikipedia.org/wiki/Dew_point