3D versus 1D temperature structure of two 3D CO5BOLD models representing HE 0107–5240 (T_eff ∼ 5090 K, log g = 2.2, [M/H]=−4.0, α enhancement +0.4), assuming different CNO abundances for computing the opacity tables: Top: A(C)=4.39, A (N)=3.78, A(O)=5.06; Bottom: A(C)=7.39, A(N)=6.78, A(O)=7.66. The orange band outlines the 3D temperature distribution constructed from 20 snapshots of the respective high-resolution model, each resampled to 144 × 144 × 200 grid points. The dashed blue lines enclose 95.5% of the data points at given Rosseland optical depth. The black solid line shows the <3D> model (the temperature of the 3D model averaged at constant τ_Ross), the black long-dashed line represents the T(τ_Ross) relation of the 1D LHD reference model atmosphere with identical stellar parameters, treating convection by standard mixing-length theory, but otherwise using the same input physics as the 3D hydrodynamical model.