Conditions during the eight observations of GJ 1214 b. Top to bottom: mean S/N per pixel for each spectrum, the progression of airmass, the relative humidity, and the absolute radial velocity shift (ΔRV) between the planet and Earth rest frame are shown. The latter consist of the sum of the barycentric velocity, which changes during the observation, and the static system velocity. The transit window is indicated as a grey shaded region, with lighter shades corresponding to ingress and egress phases. The horizontal blue shaded region in the bottom panel indicates the interval with RV shifts between −10 km s⁻¹ and +10 km s⁻¹ from the telluric rest frame, e.g. a region that is especially contaminated by telluric lines.