A.W. Rollins and co-authors present new work comparing hygrometer measurements in the UTLS in a new JGR article. Disagreements between instruments at the low mixing ratios (<10ppm) typical of this region have caused uncertainties in the description of the physical processes controlling dehydration in the tropical tropopause layer and how water vapour enters the stratosphere, as well as hindering validation of satellite retrievals. Several hygrometers were flown simultaneously during the MACPEX campaign to allow for a full intercomparison of instruments. Differences between instruments were reduced compared to some other previous campaigns, but remain non-negligible (on the order of 20% or 0.8ppm). They suggest that unrecognised errors in the quantification of instrumental background for some or all of the hygrometeors are a likely cause for these differences. Until these errors are better understood, the measurement uncertainty will continue to limit our understanding of dehydration in the tropical tropopause layer and cirrus microphysical processes. The full abstract can be found here.