Relative humidity (top panel), air mass (middle panel), and mean S/N per spectra (bottom panel) during the transit observations of HD 209458 b (left) and HD 189733 b (right). The transits occur between the vertical dashed lines. Out-of-transit open symbols represent the spectra that are not included in the analyses because of their low S/N. Open symbols during transit represent spectra that are not included due to the overlap of telluric and planet lines when the latter has a near-zero velocity in the observer’s frame.