The tiny amounts of water in the upper troposphere and lower stratosphere have enormous impact on the Earth’s climate, driving both radiative transfer and atmospheric chemistry. We propose to investigate the processes that are responsible for controlling water vapor concentrations in the upper troposphere and lower stratosphere through a series of flight measurements using a suite of already developed and built aircraft instruments. Our UAV science demonstration will produce unique measurements not made by conventional platforms, yet will minimize risk and cost by use of a small suite of existing instruments including a laser hygrometer, a microwave temperature profiler, solar radiometers, and a laser spectrometer specifically developed and built for UAV application. No instrument development is proposed. Our primary measurements will be those of HDO, H₂O, CH₄, N₂O, CO, temperature profiles, and solar flux radiometry. Our chosen UAV platform is AeroVironment’s solar powered aircraft, using flights of Helios (test flights) and Pathfinder Plus that provide cross-tropopause and vertical profiling ability important to our science demonstration. In addition, we also propose to include validation measurements of H₂O and temperature profiles for either or both of the satellite instruments AIRS and SAGE III during measurement coincidences, and thereby further demonstrate the utility of UAVs for satellite validation.