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International audienceWe present here in situ measurements obtained between 1991 and 2011 in outervortexconditions by the ELHYSA balloon-borne frost-point hygrometer. The frost-point hygrometerprofiles are used for comparisons with the satellite data from version 19 (v19) andversion 3.3 (v3.3) of the HALogen Occultation Experiment (HALOE) and theMicrowave LimbSounder (MLS) respectively. Potential Vorticity mapping is applied to all data sets to removecontributions of transient tropical intrusions and polar vortex air masses and hence ensureconsistent comparisons between the balloon and satellite observations. Our selected balloon insitu observations are too sparse to directly infer mid-latitude stratospheric time series forcontinuous comparisons with HALOE and MLS records or derive water vapour trends but canbe used to validate the satellite data. A mean difference of −0.83±1.58 % (−0.04±0.07 ppmv) isobtained between HALOE v19 data and the balloon frost-point observations (with respect toHALOE) over the 30–80 hPa altitude range. The hygrometer-HALOE differences appear timedependentas already presented in the literature. The mean difference reaches 2.80±0.96 %(0.13±0.04 ppmv) forMLS v3.3,withMLS systematically wetter than the balloon data reflectinga systematic bias between both datasets. We use our balloon data as reference to provide someinformation about the HALOE-MLS difference. From post-2000 ELHYSA-HALOE andELHYSA-MLS comparisons, we find a HALOE-MLS difference matching the expected bias,with MLS v3.3 6.60±2.80 % (0.27±0.11 ppmv) wetter than HALOE v19. From the results obtained from our balloon-satellite data comparisons, we finally discuss the issue about mergingthe HALOE and MLS data sets to provide stratospheric water vapour trends

Balloon-borne observations of mid-latitude stratospheric water vapour: comparisons with HALOE and MLS satellite data