Effect of the H₂O CIA at 175 (Z/H)_⊙ on the simulated transmission spectrum and temperature profile of K2-18b. Left: temperature profiles. Solid black: nominal model. Dotted-dashed black: nominal model at 0.8 time the nominal irradiance. Red: nominal model without H₂ O CIA. Dotted-dashed red: nominal model at 0.8 times the nominal irradiance and without H₂ O CIA. The phase diagram of H₂O is represented as dotted blue lines, the dot corresponding to the H₂O triple point. Right: transmission spectra at 0.8 time the nominal irradiation, with (black) and without (red) H₂ O CIA.