Humidity affects the atmosphere's density and speed of sound, and thus, the drag on the bullet. The effect of humidity is slight compared to temperature, but it can become a factor at long ranges. If you don't know the actual value, use the standard value of 78 percent.
There are many instruments available for measuring (relative) humidity directly. However, one method still in use measures the temperature of a dry bulb (thermometer bulb) and a wet bulb. The program can calculate humidity directly from the bulb readings. Click on the Options button and then click on the Calc Relative Hmdty button.