Fraction by mass of CO-dark H₂ gas for all sources as a function of the total hydrogen column density N_tot = N(H i) + 2N(H₂): diffuse molecular clouds (red °), ¹²CO transition molecular clouds (blue ×), and ¹³CO dense molecular clouds (black △). The curves on the left are f(CO-dark H₂) calculated from the models of Visser et al. (2009) as discussed in the text. The central curve (dashed blue line) is the 100-point means of their model shown in the right panel of their Fig. 7. The top curve (dotted red line) is the lower limit on the solutions for N(CO), while the bottom curve (solid black line) is the upper limit on N(CO) as a function of N(H₂) for the range of parameter space used in their models. On the right side we plot the few values of f(CO-dark H₂) calculated by Wolfire et al. (2010) for massive clouds for different values of metallicity relative to the solar value, Z′ = 0.5 (red line), 1.0 (blue line), and 1.9 (black line).