Apparent horizontal velocities, v_HOR, of running penumbral waves at two atmospheric layers. The results for the photospheric Fe i line (Figs. 5d–f) are shown in black, for the chromospheric Na i line (Figs. 5a–c) in red. Left panel: the apparent horizontal velocities (in km s^-1) over the first 4″ of the inner penumbra are plotted according to the observation time, t. The asterisks, diamonds, and triangles show the apparent speeds of the wave trains in sectors 1, 2, and 3. The dashed lines indicate the average apparent velocities, ⟨ v_HOR, Fe i⟩ and ⟨ v_HOR, Na i⟩ for both layers. The averages and their standard deviations are given in the figure legend. Right panel: histogram showing the number #N of velocity values v_HOR from the left panel within a 5 km s^-1 binning interval.