T_b–T_s relations calculated for a sample of representative values of T_s in the case of Fe envelopes. Different colours and markers denote different bottom-of-the-envelope densities ρ_b. The models from Gudmundsson et al. (1983) and Potekhin et al. (2015), calculated for the standard value ρ_b = 10¹⁰ g cm⁻³, are plotted for reference. The spread of the points at the same T_s (which is noticeable only in the bottom panel) corresponds to different Θ_B, with higher T_b giving a lower T_s value for the same T_b. Two values for the magnetic field, 10⁸ G (top panel) and 10¹² G (bottom panel) are shown; these values have been chosen, although comparatively low, for clarity of visualisation, as stronger fields yield very sparse plots.