aluminum oxide humidity element
From Glossary of Meteorology
A humidity-sensing element consisting of a base of aluminium, an oxide made by anodizing the base material, and an evaporated conductive coating of metal.
The base material acts as one electrode and the evaporated metal as the other, or outer electrode. An increase in relative humidity causes the impedance between the electrodes to drop. The reverse is true when the relative humidity decreases.
