A hygrometer technically very similar to the Lyman-alpha hygrometer, but with the hydrogen radiation source replaced by a Krypton glow tube. The main emission is at 123. 58 nm, and the main advantages are a longer lifetime of the Krypton tube and a better calibration stability compared to the Lyman-alpha hygrometer, which makes this instrument more suitable for continuous humidity recordings. On the other hand, the sensitivity of the Krypton hygrometer is distinctly lower; therefore it is used mainly for ground- based measurements.