V-V phase shift between the Ca II and Hα chromospheric velocities at the analyzed locations. The background shows an intensity map of the umbra and surroundings at the continuum around the Ca II 8542 Å line. The colored dots mark the analyzed locations, and the color corresponds to the V-V phase shift at 6.45 mHz according to the upper color bar. The contours indicate the number of umbral flash profiles as described in Fig. 1. The letters A-D indicate the locations illustrated in Figs. 6, 8, 9, and 10.