Water vapor (WV) plays an important role in the radiative balance on Earth since it is the principal source of infrared opacity. Its contribution to natural terrestrial greenhouse effect is of about 60 to 75% [1–3]. Additionally, simulations based on radiative-convective models and observations have demonstrated that an increase in greenhouse gases like CO2, inducing a warming of the global surface temperature, could lead to a moistening of the troposphere [4–8]. This is the specific humidity feedback or water vapor feedback. In the IPCC 6th assessment report, the combined water vapor and lapse-rate feedback (due to changes in air temperature) makes the largest single contribution to global warming, whereas the cloud feedback remains the largest contribution to overall uncertainty. Vertical profile of water vapor are an essential meteorological observation, particularly with respect to numerical weather prediction (NWP) and global climate modeling.

            Focusing on the middle atmosphere, observational studies have shown that an increase of stratospheric water vapor could lead to a warming of the mean surface temperature [9,10]. Even a small change of water vapor (less than 1 ppm) in the lower stratosphere represent an important source of the decadal variability in the surface temperature. Stratospheric water vapor has increased in the middle stratosphere at a mean rate of 0.6%/year whereas a trend is difficult to estimate nearby the tropopause due to the variability of its height and the influence of dynamic processes which modulates the abundance of water vapor. The entry of water vapor in the stratosphere is driven by numerous factors: the quasi-biennial oscillation [11–14], the strength of the Brewer-Dobson circulation [15,16], and the temperature of the tropical tropopause, deep convection, atmospheric waves… Then, the TTL has usually been excluded from trend analyses to avoid the strong variability due to tropopause dynamics. However, changes in water vapor in the UT/LS exert a greater radiative forcing than changes elsewhere [17,18]. 

            Worldwide there are more than 800 radiosonde stations launching balloon instruments twice daily, totaling > 500 000 vertical profiles of wind speed/direction, pressure, temperature, and humidity each year. These measurements are predominately used for weather forecasting. While radiosonde wind, pressure, and temperature measurements are accurate up to balloon burst (> 30 km), the relative humidity measurements often lack the precision and accuracy necessary for climate research and are most useful in the low to mid-troposphere. Stratospheric water vapor is deceptively hard to measure. In response, in the community, scientific-grade hygrometers have been developed. Intercomparison airborne campaigns have shown that  the agreement between a core group of hygrometers has improved over the past 2 decades from ±30 % or more to approximately ±5–20 % for mixing ratios under 10 ppmv [19]. Recently, in the frame of the Implementation plan 2022, the GCOS revised ECV requirements, implying more stringent objectives. Sub-kilometer vertical resolution is requested. The quality of the measurements is addressed by 3 thresholds in measurement uncertainty: Threshold (T : 500 ppbv ≈ 10%), Breakthrough (B : 250 ppbv ≈ 5%) and Goal (G : 100 ppbv ≈ 2%).

            Due to the dryness of the UT/LS, measuring water vapor in this region remains a technical challenge. Then, water vapor is the second most important ECV following temperature. At the European and international levels, water vapor is one focii of the European projects MeteoMet since 2011 (JRP ENV07 et JRP ENV58).