Altitude profile obtained with HARPS and method 3. The left vertical axis is the measured altitude, set to 23.8 km at at [4520–4540] Å. The right vertical axis is the corrected altitude, if one considers the lowest region of h close to altitude 12 km as in Fig. 12 and again h = 23.8 km at [4520–4540] Å. The horizontal dotted line represents the altitude 23.8 km.The light-blue line is a 10 Å bin superimposed on the full-resolution profile. Oxygen and water-vapour absorption spectra at arbitrary scale are shown.