psychrometer, a hygrometer composed of two similar thermometers. The bulb of one thermometer is kept wet (by means of a thin, wet cloth wick) so that the cooling that results from evaporation makes it register a lower temperature than the dry-bulb thermometer. When readings are taken simultaneously, it is possible (with the use of psychrometric tables) to determine the relative humidity and dew-point temperature of the air. A decrease in the humidity of the air brings an increase in the difference between dry- and wet-bulb temperatures, called the wet-bulb depression. See also hygrometer.