How to find Relative Humidity?
If the Temperature is 20°C and Dew point temperature is 20°C, what is the Relative Humidity?
Relative Humidity+Specific Humidity/Capacity X 100
It turns out that calculating relative humidity is a complicated busuness. You can go to this site and see if you can understand how it is calculated:
http://www.theweatherprediction.com/habyhints/186/
100 % by definition If the relative humidity is 100%, the dew point is equal to the current temperature