Ratios of the relative level populations calculated accurately by solving the statistical equilibrium equations for CO, assuming very low optical depths, and those calculated assuming thermalized level populations, R_T. See the discussion in the text. We calculated R_T for the J = 2 and J = 6 levels and for the conditions expected in our main model features A, B, C, D, and W, as indicated in the figure. The deviations from thermal equilibrium populations are in all cases <10%.