CO velocity distributions for Sun-centred apertures of various sizes, with standard deviations noted in the legends. Top: estimated spectra for the diffuse component in the survey area, with a minimum and maximum velocity dispersion. Middle: combination of the two, how they would look if observed together. The grey lines show what we estimate as the observation threshold (see main text), and we estimate the velocity standard deviations above this baseline. Bottom: standard deviations of the apertures containing the clouds alone vs the estimated total spectra. The blue line corresponds to a one-to-one relation.