From Glossary of Meteorology
- 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.