https://cdn.earthdata.nasa.gov/conduit/upload/5182/KeywordsCommunityGuide_Baseline_v1_SIGNED_FINAL.pdf draft 2025-07-25 https://gcmd.earthdata.nasa.gov/KeywordViewer/scheme/instruments/83c84b20-7593-4c2e-9e0c-7d7a51b0d39d NOAA TW Date=2025-12-05 User Id=tstevens User Note=added new instrument keyword Date=2025-12-05 User Id=system System Note=Added broader relation from NOAA TW [83c84b20-7593-4c2e-9e0c-7d7a51b0d39d] to Hydrometers [c848a856-6210-47c4-b8e3-97427f04a035] Total water is measured in situ as vapor with a Lyman-Alpha hygrometer. High ambient sample flows through a closed cell minimize the effect of trapped water. Lyman-a light (121.6 nm) photodissociates water to produce an excited OH radical. The fluorescence from this radical at 309 nm is detected with a phototube and counting system. At aircraft pressures the fluorescence signal is quenched by air which gives a signal that is proportional to mixing ratio. The Lyman-Alpha radiation produced with a DC-discharge lamp is monitored with an iodine ionization cell that is sensitive from 115 nm to 135 nm. Calibration occurs in flight by injecting water vapor directly into the ambient sample flow. 2025-12-05T17:50:54.142Z