Psychrometer. An instrument for indicating the atmospheric moisture content (q.v.). It consists of two thermometers (q.v.), the bulb of one being kept moist by a wet wick and ventilated. The wet bulb will experience evaporative cooling and show a lower temperature than the dry-bulb thermometer. The rate of evaporation (q.v.), which is related to the vapor pressure (q.v.) of the air, will be greater on dry than on humid days and hence the cooling effect will differ correspondingly and result in variable differences between the two thermometer readings. These differences in temperatures are directly proportional to the existing relative humidity (q.v.) of the surrounding air.