Temporal evolution (in min) of the relative velocities in the sunspot atmosphere. The analysis of time slices was performed for the Doppler velocities V_{Na i} (panels a)–c)) of Na i 589.6 nm and V_{Fe i} (d)–f)) of Fe i 630.15 nm in all three sectors, also shown in Fig. 3. For each sector, the azimuthal average at a radial distance X (in arcsec) from the spot center was calculated. The dashed lines mark the umbral boundary. The velocity scale ΔV_LOS ranges between ±0.6 km s^-1 (a)–c)) and ±0.3 km s^-1 (d)–f)). The black bars trace the apparent wave trains.