Thick line at right side of each panel: temperature difference, ΔT, of the line-forming region relative to its environment for the same positions as in Fig. 12. See main text for the exact definition of ΔT. Positive ΔT: the environment is hotter than the line-forming region. Lines on the left side of each panel: normalized contribution functions for the three methods indicating the heights of line formation (thick solid: 3D NLTE; thin solid: 1D NLTE, dashed: LTE).