From Glossary of Meteorology


An instrument showing qualitatively whether the air is dry or damp, usually by the change in appearance or dimensions of some substance (
see chemical hygrometer).

Hygroscopes are frequently used as home weather predictors, for example, "weather houses" in which the appearance of the "old man" or the "old woman" is determined by the twisting and untwisting of a piece of catgut in response to changes of humidity.