Histograms of the occurrence of different FWHM (top) and centroid velocity (bottom) of the different detected line components of ¹³CO J = 3 → 2. The left column is for IRDC C and the right column for IRDC F. In the FWHM histogram for IRDC C, components with centroid velocities greater than 80.5 km s^-1 are shown in light red, velocities less than 77 km s^-1 in dark blue, and intermediate velocities in white. For IRDC F, in the FWHM histogram, components with centroid velocities less than 54.5 km s^-1 are shown in blue. These threshold velocities are denoted as dashed vertical lines in the bottom panels in the corresponding colours. For the VLSR histograms, components with a FWHM less than 3 km s^-1 in IRDC C and less than 2 km s^-1 in IRDC F are shown in green. This dividing width is denoted as the vertical green line in the top panels.