Absolute value of the difference in line-centre velocities of ¹³CO (1−0) and NH₃ lines detected in the ATLASGAL sources plotted against the ammonia linewidth in the top panel. The differences of most clumps are similar to the typical NH₃ linewidth, which is indicated by the dashed red line, and smaller than the width of the ¹³CO transition labelled by the solid green line, the rms of the absolute value of the difference in ¹³CO and NH₃ (1, 1) velocities is shown by the dashed-dotted line. The bottom panel shows the relative number distribution of the ATLASGAL clumps with the velocity differences of both molecules. The ¹³CO and NH₃ velocities of most sources differ by more than the sound speed at 0.23 km s^-1.