Highly accurate and high resolution (spatial or temporal) measurements of water vapor concentration, both in field and laboratory experiments, are increasingly demanded to address contemporary research questions regarding cloud microphysics and cloud–turbulence interactions. At the Institute of Geophysics UW, we adapted the Fast Infrared Hygrometer (FIRH), an instrument employing tunable diode laser absorption, to perform humidity measurements at the turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T). The goal of the series of experiments was two-fold: (1) to evaluate the properties of FIRH under a wide range of well-defined reproducible conditions resembling those in the real atmosphere, (2) to characterize the humidity field and turbulent fluctuations of humidity inside LACIS-T for different settings of the tunnel. It is an ideal facility to test FIRH because temperature and humidity in each of the two streams entering the measurement volume can be precisely controlled, while the turbulent mixing of the streams produces fast fluctuations of temperature and humidity. On the other hand, FIRH is well-suited to resolve small-scale and quickly changing features of the humidity field inside LACIS-T because it provides high temporal resolution and its typical optical path roughly corresponds to the width of the LACIS-T measurement section. This enables contactless optical sampling from outside the tunnel which eliminates the influence of the instrument on the investigated processes.