To find the relative humidity as a percentage from a dry and wet bulb thermometer, it is necessary to record these readings using two standard thermometers and a psychrometric chart. One thermometer measures the dry bulb temperature while the other measures the wet bulb temperature.
To take the wet bulb temperature, place a wet cloth around the bulb and record the temperature reading, which will be lower than the dry air temperature. Similarly, record the dry bulb temperature reading of the air and use the chart to find the relative humidity. A psychrometric chart has the dry bulb temperatures at its base, and the wet bulb temperatures are found on a curved line at the left of the chart. The point where the two readings intersect on this chart is the relative humidity in percent.
An example to find the relative humidity on this chart is using a dry bulb reading of 80 degrees Fahrenheit and a wet bulb reading of 67 F. This will give a relative humidity result of 50 percent.