a) Ratio of the pressures supporting a clump against a collapse (total kinetic pressure) to the pressures promoting it (external and gravitational pressure) in clumps with  cm^-3. The different curves are for clumps with radii R = 0.1−2.1 pc, in steps of 0.2 pc. The dashed vertical lines indicate the interval of the observed mean extinctions (see Table 1). b) The same, but using Larson’s size-linewidth relation to calculate the kinetic energies of the clumps (i.e., Eq. (15) instead of Eq. (13)).