Vertical cavity laser hygrometer for the National Science Foundation Gulfstream‐V aircraft

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 × 10 11 molecules/cm 2 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.