Humidity strip

From Glossary of Meteorology

humidity strip

(Also called electrolytic strip.) A flat plastic strip bounded by electrodes on two sides and coated with a hygroscopic chemical compound such as lithium chloride.

The electrical resistance of this coating is a function of the amount of moisture absorbed from the atmosphere and the temperature of the strip. Humidity strips have been used in radiosondes, but are replaced today by thin-film capacitors or carbon-film hygrometer elements.
See dew cell.