HYGROMETER

instrument used in meteorological science to measure the humidity, or amount of water vapour in the air. The hygrometer is especially important in the field of micrometeorology, in which it is necessary to make rapid measurements in thin layers of air of large changes in relative humidity. Three major types of hygrometers are used to indicate relative humidity. (Absolute humidity is determined from the relative humidity, if needed.) Mechanical hygrometers make use of the principle that organic substances (particularly finer substances such as goldbeater's skin and human hair) contract and expand in response to the relative humidity. Contraction and expansion of the hair element (blond hair is always used) in a mechanical hygrometer causes the spring to move the needle on the dial. Electrical hygrometers measure the change in electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the relative humidity changes. Other hygrometers sense changes in weight, volume, or transparency of various substances that react to relative humidity. Dew-point hygrometers typically consist of a polished metal mirror that is cooled at a constant pressure and constant vapour content until moisture just starts to condense on it. The temperature of the metal at which condensation begins is the dew point. The psychrometer (q.v.) is a hygrometer that utilizes two thermometersone wet-bulb and one dry-bulbto determine humidity through evaporation.