hygrometer

A meteorological instrument that measures the relative humidity of the air. There are several types, depending on the system used to detect variations in the water vapor content of the air. The most common of all is the hair hygrometer, invented by the Swiss naturalist Horace-Bénédict de Saussure (1740-1799) in 1783. It bases its measurements on the variations in length undergone by a few degreased hairs, given their high sensitivity to changes in ambient humidity. The popular "weather friar" belongs to this category. Another type of hygrometer fairly widely used in weather observatories is the double-thermometer type, also known as the psychrometer. To these are added the dew-point type - such as the Daniell or Regnault type -, the electrical-resistance type, and the absorption type, which incorporates a hygroscopic chemical substance that traps the water vapor contained in a given volume of air; through weighing, the humidity of the original air is determined, before undergoing drying.