Hα four-panel diagram for the RBE at x = 10.7 Mm in Fig. 4. The plotted quantities, as functions of frequency and height, are indicated in the upper-left corner of each subplot and plotted in a grayscale. The vertical component of velocity as a function of height is shown in all panels (solid white line), with upward velocity being positive. The red line marks the τ = 1 position as a function of frequency and height. Planck and total source function are plotted in the upper-right panel with dotted and dashed lines, respectively. The line profile is shown with a solid white line in the lower-right panel.