From Glossary of Meteorology
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Any instrument that measures the water vapor content of the atmosphere.

There are six basically different means of transduction used in measuring this quantity and hence an equal number of types of hygrometers. These are 1) the psychrometer, which utilizes the thermodynamic method; 2) the class of instruments that depends upon a change of physical dimensions due to the absorption of moisture (
see hair hygrometer, torsion hygrometer, goldbeater's-skin hygrometer); 3) those that depend upon condensation of moisture (
see dewpoint hygrometer, frost point hygrometer); 4) the class of instruments that depend upon the change of chemical or electrical properties due to the absorption of moisture (
see absorption hygrometer, electrical hygrometer, carbon-film hygrometer element, dew cell); 5) the class of instruments that depend upon the diffusion of water vapor through a porous membrane (
see diffusion hygrometer); and 6) the class of instruments that depend upon measurements of the absorption spectra of water vapor (
see spectral hygrometer).

Middleton, W. E. K., and A. F. Spilhaus 1953. Meteorological Instruments. 3d ed., rev., . Univ. of Toronto Press, . 105–116.

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