A vertical cavity diode laser hygrometer using two absorption lines near 1854 nm has been developed for the National Science Foundation Gulfstream-V aircraft. This instrument operates from the surface to the lower stratosphere, measuring 6 orders of magnitude in water vapor concentration. The optical system consists of an open-path multiple-pass cell mounted on an aerodynamic pylon. The self-operating hygrometer reports concentration in real time at 25 Hz and uses a novel approach for fitting the data with minimal correction terms for changes in ambient pressure and temperature. The instrument intercompares with existing research grade hygrometers in the 2%–10% range. A minimum detection limit of 3.6 × 1011 molecules/cm2 or 0.08 ppmv at 15 km altitude conditions is achieved. The design rationale, operation, and flight performance of the hygrometer are described in this work.