Temperature distribution with Rosseland optical depth at the surface of a metal-poor red giant at  [Fe/H]  =  −2. Left panel: predicted distribution from a representative snapshot of the simulation computed solving the radiative transfer equation self-consistently for a source function with a coherent scattering term (standard case). Blue curve: mean stratification from the corresponding simulation computed assuming a Planckian source function and excluding scattering from extinction in optically thin layers. Right panel: corresponding distribution from a simulation assuming a Planckian source function with scattering as true absorption (mean stratification shown in red). Dashed lines (both panels): mean stratification from the standard-case simulation. All snapshots were taken 28 h after the start of the simulations. See Fig. 2 for more details.