A Psychrometer measures relative humidity. Two thermometers are used in a Psychrometer. A wet cloth covers the end of one of the thermometers. Water evaporates from this cloth, causing the temperature on that thermometer to be different from the other. If the surrounding air is dry, more moisture evaporates from the wick, cooling the wet-bulb thermometer more so there is a greater difference between the temperatures of the two thermometers. If the surrounding air holds as much moisture as possible – if the relative humidity is 100% – there is no difference between the two temperatures.