A small light-weight in-house made miniature chilled-mirror hygrometer (CMH) for fixed wing UAS (unmanned aircraft system) is presented, with its features and limitations. Therefore, first measurements of the CMH equipped on the small research UAS of type MASC‑3 (multi-purpose airborne sensor carrier) operated by the University of Tübingen are shown. A comparison against a very accurate state of the art capacitive industrial humidity sensor (SHT31) is done. The sensor consists of a TEC (thermoelectric cooler) covered by a gold mirror. The TEC is controlled by a commercially available microprocessor with an on-board PID (proportional-integral-derivative) controller. The results of the CMH measurements are in good agreement with the industrial-made capacitive sensor. The absolute accuracy of the measured dew point temperature by the CMH is in the range of ±0.2 K. Spectra show evidence that the CMH is capable to measure turbulent humidity fluctuations in the atmosphere with a temporal resolution of up to 10 Hz. Such a fast humidity sensor aboard a small UAS has the potential to study humidity fluxes in the surface layer over complex terrain, behind wind energy converters and humidity variations over land and sea surfaces in general.