We introduce a new retrieval protocol for chilled mirror hygrometer measurements that enables balloon-borne frost point measurements in the upper troposphere/lower stratosphere of unprecedented accuracy under conditions of rapidly changing humidity. Chilled mirror hygrometers measure the frost point (or dew point) by quantifying the saturation ratio of the air with respect to the condensed phases of water (ice or liquid). To this end, they attempt to determine the thermodynamic equilibrium of the mirror condensate with the vapor phase by measuring the mirror reflectance, which changes with the amount of condensed material. In the rapidly changing environment along the balloon trajectory, however, the adjustment of the mirror temperature to the new equilibrium point may lead to frequent, damped overshoots or nonequilibrium errors. For the Cryogenic Frost Point Hygrometer (CFH), a balloon-borne chilled mirror instrument of reference quality, we (i) identify points in time along the sounding profile when the mirror is in equilibrium with the gas phase, which we term “Golden Points”, and (ii) correct the measurements under nonequilibrium conditions between these Golden Points. For (i), we identify the points where the suitably smoothed mirror reflectance assumes an extreme value, i.e. a maximum or a minimum. At these extreme points, the CFH mirror temperature represents the frost point with an accuracy better than 0.2 K (resulting from the uncertainties of the mirror temperature sensor and of the precise timing of the Golden Points along the sounding profile). These accurately determined frost points can be used to detect and correct offsets, biases and time lag errors in other humidity sensors flown together with CFH on the same balloon payload, such as the FLASH-B fluorescence hygrometer or the thin-film capacitive hygrometer of the Vaisala RS41 radiosonde. From the surface to approximately 28 km, a frost point uncertainty of 0.2 K corresponds to less than 4 % uncertainty in H2O mixing ratio (including the uncertainty of 0.3 hPa in the GPS-based pressure measurement of the RS41 radiosonde), provided there is no outgassing from the balloon or the instrument components. For (ii), we compute the time-derivative of the mirror reflectance, which is proportional to the nonequilibrium error. The proportionality factor is related to a property of the mirror condensate, which we term “morphological sensitivity”, and allows correction of the CFH nonequilibrium data. The sensitivity constant is determined using an a-priori reference, such as the RS41 radiosonde humidity measurements after they have been time-lag and bias-corrected by means of (i). Alternatively, under suitable circumstances, the morphological sensitivity can also be derived from matching ascent and descent data, or from closely spaced Golden Points. Based on 70 nighttime CFH-RS41 tandem flights, the nonequilibrium analysis suggests that 18 % of the about 23 000 measurement points in the upper troposphere are associated with errors in the frost point of more than 0.2 K, 10 % with > 0.3 K, 4 % with > 0.5 K, and 1 % with > 1 K. Most profiles (68 %) have errors > 0.2 K in at least 5 % of their measurements. While these errors are consistent with the reported accuracy of the CFH instrument, there are occasions (∼ 0.1 % of the measurement points and ∼ 5 % of the flights) when the mirror temperature deviates by more than 3 K from the true atmospheric frost point or > 40 % error in H2O mixing ratio in the tropopause region. Large errors of CFH are due to suboptimal control of the mirror temperature in certain measurement scenarios (such as large mixing ratio changes in the atmosphere or the presence of a coarse ice film on the mirror). We estimate that the nonequilibrium correction removes over 80 % of large nonequilibrium errors, which is superior to the low-pass filtering and time-lag correction techniques found in the literature. In particular, layers of extreme cold bias (< −3 K) can be identified and corrected, which is important for atmospheric trend detection. The study highlights that the output data of a chilled mirror hygrometer is to be considered from two perspectives, the mirror temperature and the mirror reflectance. Using both quantities enables frost point measurements that meet the target established by the World Meteorological Organization in 2024 for reference instruments used to measure water vapor in the atmosphere.
