HAI is a fully autonomous, airborne Hygrometer for Atmospheric Investigations for simultaneous gas-phase/total H2
O detection on the HALO aircraft. HAI employs 1st-principles, direct, Tunable Diode Laser Absorption Spectroscopy (dTDLAS) for calibration-free, absolute H2
O detection. HAI simultaneously measures at 1.4/2.6 µm and in closed-/open-path configuration, covers a H2
O range of 1–40 000 ppmv at up to 1.4 ms time resolution and achieves precisions of 0.18/0.055 ppmv at 1.4/2.6 µm.