An instrument used to measure humidity. It consists of two thermometers exposed side by side, one of which (the dry bulb) is an ordinary glass thermometer, while the other (the wet bulb) has its bulb covered with a jacket of clean muslin that is saturated with distilled water prior to an observation. The temperature measured by the wet-bulb thermometer is generally lower (due to evaporation of water from the wet bulb) than that measured by the dry bulb. The difference in the temperatures is a measure of the humidity of the air; the lower the ambient humidity, the greater the rate of evaporation and, consequently, the greater the depression of the wet-bulb temperature. The size of the wet-bulb depression is related to the ambient humidity by the psychrometric formula. See aspiration psychrometer, Assmann psychrometer, sling psychrometer.