Temperature dependence of the high-resolution transmission spectrum of a hot Jupiter. The temperature varies between 1200 and 2300 K in steps of ~200 K (from light to dark). Left panel: zoom on one line of the sodium doublet. The contrast of the sodium feature in the hottest atmosphere (purple) is approximately twice its contrast in the coldest atmosphere (light green). This behaviour is explained through the linear dependence of atmospheric signal on temperature through the scale height. Right panel: zoom on a region of the water band. Even if in general the hottest transmission spectrum has a higher contrast compared to the coldest one, the dependence on temperature is non-linear. Some specific lines are actually stronger in a colder atmosphere (e.g. close to 11 107.5 Å).