Erica's Answer:There are 2 thermometers on a psychrometer: the dry bulb and the wet bulb. In order to measure the relative humidity, a piece of cloth surrounding the wet bulb thermometer is moistened and re-attached to the wet bulb. Then, the wet bulb thermometer is dried by swinging it in the ambient air. The evaporation of the wet bulb will cool the cloth down, thereby lowering the temperature measurement on the wet bulb thermometer. By looking at the difference between the wet bulb and dry bulb temperatures, you can get an idea of the relative humidity. The smaller the difference in temperature, the higher the relative humidity is.