Brunamonti et al. report a synthesis of results from the AquaVIT-4 project, providing a comparison of state-of-the-art atmospheric hygrometers for use in conditions found in the upper troposphere and lower stratosphere. Accuracy of hygrometers in these dry conditions is fundamental to assessment of ice microphysics, which controls the formation of cirrus and limits the transport of water vapor into the stratosphere. Substantial differences in the measurements reported in this region by various hygrometers has historically motivated a continued set of campaigns at AIDA and in the atmosphere, to assess the skill of research hygrometers.

This article is excellent. The experiment and analysis are well done, the figures are high quality and text is very well written. I essentially only have minor editorial comments for the authors to address, in addition to a couple of potential small changes that they could consider for the content and analysis.

Page7 Line5: Suggest to mention what kind of data acquisition mode is used for ALBATROSS, e.g. scanning with DC or 2f modulation, etc?

Page 8 Line 6: Suggest instead of saying “25+ years” state the first year that it was operated.

Page 9 lines 18 – 27: Is this paragraph really needed? As I understand it, this is summarizing results that are already reported in Ghysels et al 2024. The large differences stated of “+/- 23.6%” and “about 30%” I assume are differences in the air that was measured by the instruments on those experiments and not due to accuracy problems with the instruments, but this isn’t fully explained here. I would either provide more detail or remove that part.

Page 13 Line 15: need to delete word “of”

Figure 2: Recommend using a discrete colorbar with ~5 degree intervals.

Page 14 Line 7: “instrument” -> “instruments”

Page 26 Line 8: “Evaporates” should be changed to “sublimates”. However, I’m having trouble understanding that this is really the mechanism that results in a loss of ice. Is it rather that the ice crystals sediment to the bottom of the chamber? It doesn’t make sense that the ice cloud would sublimate while it is supersaturated.

Page 26: Does the result in Figure 9 imply a positive bias in APicT and is it worth making a comment about how that might impact the comparisons shown previously where that hygrometer was the reference? Or is it rather the case that the uncertainty range shown for 100% RHi in Figure 9 is considered uniform such that the correct value could be anywhere in that +/-5% range with equal probability?

One other comment I have is on the lack of substantial discussion or analysis about instrument precision. Precision is discussed a bit towards the end of the manuscript, but is not quantitatively summarized e.g. in Table 1 or assessed elsewhere. This paper would be a useful venue for comparison of the precision of the hygrometers as well as accuracy so I suggest that the authors consider addressing this somewhere. One suggestion if the authors do this is that some markers indicating the observed precision could be added in e.g. Figure 8, or a separate figure could be generated comparing the precision of the different instruments in this relevant P, H2O space.