Owing its role as “gateway” to the global stratosphere, knowledge of the tropical tropopause layer with respect to its water budget is important in order to understand the radiative and chemical properties not only of this layer itself, but of the entire stratosphere, including its impact on global climate. Previous measurements suggest that these measurements are very difficult, and no accepted instrumental “gold standard” exists. In particular, instrumental uncertainties have prevented to properly judge to which degree observed persistent extremely high saturation ratios
over ice (RHice >> 1.5) within cirrus clouds are real or only an instrumental artifact. Here we show that humidity measurements within cirrus clouds allow to estimate the measurement uncertainty of each instrument, owing to the restoring force that the ice surfaces have on the H2O in the gas phase. The measurements of most instruments can be explained as superposition of this restoring force and small-scale temperature fluctuations, allowing to explain even data points with extreme super‐ or subsaturation. In general, this demonstrates progress in measurement technology applicable to the adverse conditions of the tropical tropopause layer. On the other hand, this analysis makes clear that future improvements in our quantitative understanding of the freeze‐drying process will only be possible if the remaining measurement uncertainties of RHice of ca. 20% can be overcome. Finally, I will confront this analysis with some results from the ballooning activities presently ongoing here in Nainital.