Disclaimer: This material is being kept online for historical purposes. Though accurate at the time of publication, it is no longer being updated. The page may contain broken links or outdated information, and parts may not function in current web browsers. Visit https://espo.nasa.gov for information about our current projects.


Diode Laser Hygrometer (DLH)


Operated By: 

The DLH has been successfully flown during many previous field campaigns on several aircraft, most recently ATom, KORUS-AQ, and SEAC4RS (DC-8), POSIDON (WB-57), CARAFE (Sherpa), DISCOVER-AQ (P-3), and ATTREX (Global Hawk). This sensor measures water vapor (H2O(v)) via absorption by one of three strong, isolated lines in the (101) combination band near 1.4 μm and is comprised of a compact laser transceiver mounted to a DC-­8 window plate and a sheet of high grade retroflecting road sign material applied to an outboard DC‐8 engine housing to complete the optical path. Using differential absorption detection techniques, H2O(v) is sensed along the 28.5m external path negating any potential wall or inlet effects inherent in extractive sampling techniques. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H2O(v) concentration based on the differential absorption signal magnitude, ambient pressure, and temperature, and spectroscopic parameters that are measured in the laboratory. Preliminary water vapor mixing ratio and derived relative humidities are provided in real-time to investigators aboard the DC-8.

Instrument Type: 
Point(s) of Contact: 
Glenn S. Diskin (POC; PI), Glen Sachse (Prev PI), Joshua P. DiGangi (Co-I), John B. Nowak (Co-I), James Flynn (Co-I), Sergio Alvarez (Mgr)