From Glossary of Meteorology
An instrument used to measure the amount of dew formed on a given surface.
One type consists of a hemispheric glass vacuum cup exposed to the atmosphere. Dew forming on the glass surface automatically collects in the bottom of the cup and is weighed at the end of the exposure period. Another form of drosometer, the Duvdevani dew gauge, consists of a block of wood with its surface treated in such a manner that dew forms in characteristic patterns. Photographs are supplied with each instrument to enable the observer to match the dew formation with a set of standards corresponding to a dew "fall" of from 0.01 to 0.45 mm (0.0004–0.018 in.).