Zooms on the HARPS altitude profile from method 3 shown in Fig. 21. The left vertical axis is the measured altitude, set to 23.8 km 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. Oxygen and water-vapour absorption spectra at arbitrary scale are shown. Water vapour is not detected above 16 km, suggesting that the amount of water vapour is too low to absorb enough to be visible at this altitude. The sodium layer Na I is not detected.