The mean ¹3CO spectra of the IRDCs of our sample. The spectra have been averaged over the A_V > 7 mag box (see Sect. 3.2 for the detailed definition). The dotted vertical lines indicate the velocity interval chosen to represent the cloud. The red line shows a fit of a Gaussian to this velocity interval. The blue line shows another Gaussian fit, performed over the interval v_peak − 1.5σ,v_peak = 1.5σ where σ is the dispersion from the first Gaussian fit. The dispersions are shown in the panels. The third dispersion value, σ_d gives the standard deviation of the data within the chosen velocity interval.