From Glossary of Meteorology
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  1. Pertaining to a marked ability to accelerate the condensation of water vapor; in general usage, the ability of a crystalline solid, (salt, brown sugar) to absorb vapor but at such a low rate under most conditions that it does not dissolve completely.

    In meteorology, this term is applied principally to those condensation nuclei composed of salts that yield aqueous solutions of a very low equilibrium vapor pressure compared with that of pure water at the same temperature. Condensation on hygroscopic nuclei may begin at a relative humidity much lower than 100% (about 76% for sodium chloride); below this value particles remain dry. There is often a hysteresis such that particles remain liquid as the relative humidity falls and are present as a supersaturated solution. On so-called nonhygroscopic nuclei, which merely furnish sufficiently large (by molecular standards) wettable surfaces, relative humidity of nearly 100% is required to cause condensation. “Damp haze” is formed of hygroscopic particles in the process of slow growth in relatively dry air as it cools.

  2. Descriptive of a substance, the physical characteristics of which are appreciably altered by effects of water vapor.

    The hygroscopicity of certain materials has been advantageously utilized in humidity measurement and control devices, for example, the hair element of a hair hygrometer.