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Instrument: HYGROMETERS
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Description
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).

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

In, American Meteorological Society, Glossary of Meteorology online:
http://amsglossary.allenpress.com/glossary