Dew point is the temperature the air needs to be cooled to reach 100% relative humidity. At this point, the air can no longer hold water as a gas. As the temperature drops further water drops out of the atmosphere in the form of precipitation, fog and/or condensation forming on the surface of objects exposed to the atmosphere.
If you are seeking to find an estimate of how humid/dry the air will feel, dew point is much better than relative humidity.
What is relative Humidity?
Relative humidity is the amount of humidity there is in the air relative to the maximum amount there could be. It is expressed as a percentage.
How to Calculate Dew Point
To calculate dew point, take the air temperature and the relative humidity and run those values through a formula with a common set of values. This particular calculator uses the set of values from the 1974 Psychrometry and Psychrometric Charts, as presented by Paroscientific.
In the below formula, a is the air temperature, h is the humidity and result is the final output.
A = 17.27 B = 237.7 alpha = ((A * a) / (B + a)) + math.log(h/100.0) result = (B * alpha) / (A - alpha)