Difference in temperatures, T_diff, shown in color, as defined in Eq. (5), at a pressure where τ_m = 10⁻³ (around 10⁻⁶ bar) and the surface, for different hydrogen content and equilibrium temperature, T_eq. Positive values indicate inverted PT profiles (presence of thermal inversion). Negative values indicate non-inverted PT profiles (absence of thermal inversion). The shift between the two regimes does not occur at the same hydrogen content for different equilibrium temperatures (hotter planets require more hydrogen to fall into the non-inverted category). This is indicated by the (fitted) red line, for which the formula is given in Eq. (6).