wet-bulb thermometer A thermometer, the bulb of which is kept moist by a thin cloth (e.g. muslin) bag connected by a wick to a bath of clean (preferably distilled) water. As long as the air is not saturated, evaporation from the muslin keeps the wet-bulb thermometer at a lower temperature than the dry-bulb thermometer beside it, with which its readings are compared. The depression of the wet-bulb temperature gives a measure of the saturation deficit, from which the relative humidity and dew-point can be calculated.

wet-bulb thermometer A thermometer, the bulb of which is kept moist by a thin cloth (e.g. muslin) bag connected by a wick to a bath of clean (preferably distilled) water. As long as the air is not saturated, evaporation from the muslin keeps the wetbulb thermometer at a lower temperature than the dry-bulb thermometer beside it, with which its readings are compared. The depression of the wet-bulb temperature gives a measure of the saturation deficit, and so of the relative humidity of the air.