FISH
Fast In-situ Stratospheric Hygrometer

The Fast In-situ Stratospheric Hygrometer (FISH) is an in situ airborne or balloonborne closed-path hygrometer. It uses the Lyman-alpha photofragment fluorescence technique to measure water vapor in the upper troposphere and lower stratosphere. It consists of a Lyman-alpha radiation source (121.6 nm) to excite OH molecules, which emit radiation in the 285-330 nm range. The emitted radiation is detected by a photomultiplier tube (PMT), which can be used to determine the water vapor mixing ratio. FISH has a measurement frequency of 1 Hz and a detection limit of 0.18-0.13 ppmv.