This site was constructed by me, Scott Shilala, with help from the poultry hobbyist community, and support from my wonderful wife, Kelly Jo.
In order to arrive at Relative Humidity, you'll need 2 readings.
The first reading comes from your plain old Dry-Bulb Thermometer.
The second reading comes from your wet bulb thermometer.
Find the two values in their proper column, and read the relative humidity value where they intersect.
You now have your relative humidity reading.