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Abstract An extensive set of in situ water vapor (H 2 O) data obtained by the IAGOS‐CARIBIC passenger aircraft at 10–12 km altitude over 8 years (2005–2013) is analyzed. A multifaceted description of the vertical distribution of H 2 O from the upper troposphere (UT) via the extratropical tropopause mixing layer (exTL) into the lowermost stratosphere (LMS) is given. Compared to longer‐lived trace gases, H 2 O is highly variable in the UT and exTL. It undergoes considerable seasonal variation, with maxima in summer and in phase from the UT up to ~4 km above the tropopause. The transport and dehydration pathways of air starting at the Earth's surface and ending at 10–12 km altitude are reconstructed based upon (i) potential temperature ( θ ), (ii) relative humidity with respect to ice (RHi), and (iii) back trajectories as a function of altitude relative to the tropopause. RHi of an air mass was found to be primarily determined by its temperature change during recent vertical movement, i.e., cooling during ascent/expansion and warming during descent/compression. The data show, with great clarity, that H 2 O and RHi at 10–12 km altitude are controlled by three dominant transport/dehydration pathways: (i) the Hadley circulation, i.e., convective uplift in the tropics and poleward directed subsidence drying from the tropical tropopause layer with observed RHi down to 2%; (ii) warm conveyor belts and midlatitude convection transporting moist air into the UT with observed RHi usually above 60%; and (iii) the Brewer–Dobson shallow and deep branches with observed RHi down to 1%.
Zahn et al. (Tue,) studied this question.