A Frost-Point Hygrometer for Horizontal Soundings of the Lower Stratosphere

Abstract A new kind of frost-point hygrometer has been developed for horizontal soundings of the lower stratosphere. The cooling source, using the radiative cooling of a blackbody during the night, is able to cool the mirror down to −90°C. The frost deposit is detected with an optical sensing unit associated with a regulating loop controlling the mirror heating to keep the frost thickness constant. The equipment is presented and described and the results of the first flight performed in 1976 are given and discussed.