<p>A narrow-band optical hygrometer FIRH (Fast Infrared Hygrometer, Nowak et al., 2016), based on absorption of laser light at wavelength &#955;=1364.6896 nm was used for contactless measurements of humidity inside the measurement volume of LACIS-T (turbulent Leipzig Aerosol Cloud Interaction Simulator, Niedermeier et al., 2020). LACIS-T is a multi-purpose moist-air wind tunnel for investigating atmospherically relevant interactions between turbulence and cloud microphysical processes under well-defined and reproducible laboratory conditions. Main goals of the experiment were:</p><p>1) characterization and evaluation of the FIRH hygrometer in controlled conditions,</p><p>2) characterization of fast turbulent humidity fluctuations inside LACIS-T.</p><p>&#160;</p><p>Collected results indicate, that FIRH can be used to characterize turbulent fluctuations of humidity in scales of tens of centimeters with the temporal resolution of 2 kHz and presumably more. Interestingly, scanning of LACIS-T measurement volume indicated existence of turbulence and wave-like features for the investigated measurement setup in its &#160;central part, where air streams of different thermodynamical properties, yet the same mean velocity mix intensively. , However, the setup for cloud measurements include an additional flow (i.e., an aerosol flow) in the central part strongly reducing the wave-like features. In other words, cloud process studies are most likely unaffected by these features.</p><p>Finally, the experiments proved that contactless measurements of humidity conducted from outside the measurement volume of LACIS-T are useful, on condition of corrections of glass window transmission and interferences.</p><p>&#160;</p><p>Niedermeier, D., Voigtl&#228;nder, J., Schmalfu&#223;, S., Busch, D., Schumacher, J., Shaw, R. A., and Stratmann, F. (2020): Characterization and first results from LACIS-T: a moist-air wind tunnel to study aerosol&#8211;cloud&#8211;turbulence interactions, Atmos. Meas. Tech., 13, 2015-2033, doi:10.5194/amt-13-2015-2020.</p><p>Nowak J., Magryta P., Stacewicz T., Kumala W., Malinowski S.P., 2016: Fast optoelectronic sensor of water concentration, Optica Applicata, vol. 46(4) , pp. 607-618 , doi: 10.5277/oa160408</p>