Abstract. We introduce a new retrieval protocol for chilled mirror hygrometer measurements under rapidly changing humidity conditions that enables balloon-borne frost point measurements in the upper troposphere/lower stratosphere of unprecedented accuracy. Chilled mirror hygrometers measure the frost point (or dew point) of air by quantifying the degree of saturation of the air with respect to the condensed phases of water (ice or liquid water). 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 leads 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 true 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 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. In the middle stratosphere (~ 28 km), a frost point uncertainty of 0.2 K corresponds to < 4 % uncertainty in H₂O mixing ratio (including the 0.3 hPa uncertainty of the RS41 radiosonde GPS-based pressure measurement), assuming the absence of degassing from the balloon or from instrument components. At lower altitudes, the uncertainty is even less. 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). Using 70 nighttime CFH-RS41 tandem flights, we find that the deviations from equilibrium of CFH are typically less than 0.5 K, which corresponds to less than 10 % error in H₂O mixing ratio in the tropopause region. While this is consistent with the reported accuracy of the CFH instrument, there are situations when the mirror temperature deviates significantly from the true atmospheric frost point, exceeding 3 K (or > 40 % error in H₂O mixing ratio) in the tropopause region. Such 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 the nonequilibrium error, which is superior to the low-pass filtering and time-lag correction techniques found in the literature. This procedure paves the way for frost point measurements that meet the requirements for H₂O mixing ratios better than 4 % (at 2σ) set by the World Meteorological Organization in 2023 as target for reference instrumentation measuring water vapor in the atmosphere.