From AMS Glossary
The process of liquefaction of a solid by dissolution in solution resulting from absorbed water vapor.
For atmospheric processes, the uptake of water by substances and dissolution at relative humidity generally present under atmospheric conditions. For example, LiCl (used to measure relative humidity by conduction current in older radiosondes) deliquesces at relative humidity in excess of 15%. The term hygroscopic refers to a similar process, although taking place at a higher relative humidity and a slower rate. For example, bulk sodium chloride is hygroscopic and dissolves slowly in relative humidity in excess of 76%.