The Variable-Path Lyman-Alpha Hygrometer and Its Operating Characteristics

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Abstract
A hygrometer capable of making in situ humidity measurements with good accuracy and excellent time-space resolution is described. Based on the absorption of Lyman-alpha radiation by water vapor, the instrument uses a variable-path technique for achieving long-term stability and is characterized by 12 ms response time, 5% accuracy, 1 cm typical sensing path, and an operating range of −35° to +30°C dew point. The hygrometer is now operational on NCAR aircraft.