Statistics of the excitation temperature (left), ¹³CO (1–0) opacity (middle), H₂ column density and surface density derived from ¹³CO (1–0) and labeled at the bottom and the top side (right) for the three molecular clouds: the 15 km s^-1 cloud (open blue histogram), the 27 km s^-1 cloud (filled green histogram), and the 50 km s^-1 cloud (line-filled red histogram). In the left panel, the black dashed line marks T_ex = 4.5 K, which corresponds to a ¹²CO (1–0) peak temperature of 1.5 K (~3σ, see Sect. 2). In the middle panel, the black dashed line marks τ₁₃ = 0.5. In the right panel, the black dashed line represents the median mass surface density of molecular clouds in the Galactic Ring Survey (Heyer et al. 2009).