Comparison of velocity and entropy distribution in two-dimensional convection using the optically thick approximation with a radiative upper boundary condition (upper panel) and radiative transfer (lower panel). In both cases we have Pr = 100 and , corresponding to Ra = 3.6 × 10⁴. In the lower panel, the dashed line gives the contour τ = 1. In order to compare similar structures in the two plots, we have extended the color table of the lower panel to slightly more negative values of s and have clipped it at high values, which are dominated by the strong increase of s above the τ = 1 surface.