An instrument has been developed for the measurement of water vapor in the troposphere and lower stratosphere. This instrument, dubbed the NASA Langley/Ames Diode Laser Hygrometer (DLH), has been flown on 10 missions aboard NASA's DC-8 aircraft. The DLH utilizes an open-path, double-pass configuration, where the path is defined on one end by a laser transceiver mounted on the interior of a modified window panel, and on the other by a panel of retroreflecting material mounted on the DC-8's outboard engine nacelle. The DLH operates on one of two spectral absorption lines in the 1.4 μm spectral region, in a wavelength-modulated (WM) mode, with the laser locked to the center of the absorption line encountered in a reference cell. The spectral line used is determined by the local conditions - a weak line is used at low altitudes and a stronger one at high altitudes - and is changed at various times during a flight by the operator. Signal detection is accomplished by demodulating the return signal at twice the driving frequency (2F detection). The returned laser power (DC) is also measured. The DLH is calibrated in the laboratory at various combinations of pressure and water vapor density. From the calibration data and a multiparameter spectral model, a set of coefficients is developed, and these coefficients are used to convert the measured 2F/DC ratio, along with local temperature and pressure (which are measured by separate instruments aboard the aircraft), to water vapor mixing ratio.