A new instrument for in situ measurements of atmospheric water vapor from aircraft platforms has been developed based upon near-infrared turnable diode laser sources operating near 1.37 μm. The spectrometer features a unique open-path, multipass (Herriott) cell for true in situ monitoring of water vapor concentrations with precision levels exceeding those of existing Lyman α and frost point hygrometers. External sampling outside of the aircraft boundary layer minimizes ambiguities in the measured water vapor abundances. Variable spectrum acquisition rates up to 10 Hz provide fast temporal resolution free from sampling or flow rate limitations. In its current configuration the instrument operates from the right wing pod of the NASA ER-2 research aircraft and is optimized for measurements in the upper troposphere and stratosphere (to 20 km). Measurement precision is ¿0.05 ppmv in the stratosphere for a 2-s measurement integration period. The flight-ready instrument weight is 8.2 kg, and power consumption, exclusive of structural heaters, is 7.5 W. ¿ 1998 American Geophysical Union