Panel a: forcing parameter b of eight molecules, assuming T_kin = 40 K for the 15 velocity components roughly sorted by their distance to the Galactic centre. Panel b: mean (left) and median (right) values for each velocity component. Panel c: mean (left) and median (right) values for each sub-sample of clouds, from left to right: Galactic centre, 3 kpc arm, 4 kpc arm, Scutum arm, Sagittarius arm. The uncertainties represent the standard deviation for the mean and the corresponding percentiles for the median. The dashed line in each panel marks the median value of all data points shown in panel a. The velocities of the components belonging to Categories I and II are colouredin grey and black, respectively.