Displayed are the second moments as a function of the radial velocity difference of two gas columns with the same velocity dispersion but different N_HI. 1:1 denotes the situation of a N_HI ratio of unity, 1:10 and 1:100 even higher column density ratios. The higher the contrast of both superimposed column densities, the larger the necessary separation in radial velocity for a significant separation of both clouds in M₂.