frost point hygrometer

https://doi.org/10.1351/goldbook.F02536
Instrument in which the sample is passed over a cooled surface. The temperature at which frost forms on it is a function of the water content of the gas passing over the surface.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2195 (https://doi.org/10.1351/pac199062112167)