in the psychrometric chart given below, read up from 50 degC dry bulb temperature to the 20% RH curve, then follow the horizontal line from that point to the left to the outermost curve on the chart. If you don't go blind following this chart (click to enlarge it) you'll see that ...
Determine the dew point using a graph, psychrometric slide rule, calculator or psychrometric tables. Plug in the temperature readings from both the wet and dry thermometers to find relative humidity. Step 5 Apply formulas to find relative humidity. Convert Fahrenheit to Celsius by using the formula...