Airborne measurements of humidity using a single-mode Pb-salt diode laser

Instrumentation for the measurement of ambient humidity, incorporating a liquid nitrogen-cooled tunable lead-salt laser and a reduced pressure multipass absorption cell, was installed on the Lockheed L-1011 advanced research aircraft and tested on four flights. Humidity measurements taken over altitudes ranging from 0.76 to 10.7 km were compared to results obtained with a commercial dew point hygrometer. The flight tests showed generally good agreement between the two devices to within the estimated accuracies of the instruments. At altitudes above 8 or 9 km the hygrometer is not reliable, whereas the diode laser system exhibited very good sensitivity at the highest altitudes attained. A time response of better than 0.1 s was demonstrated. We believe that a compact diode laser instrument can ultimately measure atmospheric water vapor concentrations over a wide altitude range with an accuracy of better than 3%.