In our group we've developed a photoacoustic spectroscopy based hygrometer system to be used for airborne measurements. The system is a dual channel one so in case of using a proper sampling system it can measure water vapor concentration and total water content (water vapor plus ice crystals or liquid water droplets after evaporation) simultaneously. It can measure concentrations in the 0.5-60000 ppmV concentration and 50-1100 mbar pressure ranges with ~1 second response time. If you are interested in using this system for measurements than please contact me in private.