Top panel: structure of the HI/H₂ transition predicted by Meudon model as function of depth. Densities of H and H₂, with vertical line that indicates the transition defined as n(H) = n(H₂). The local emissivity of each gas layer is shown for four representative H₂ lines. Bottom panel: temperature and cumulative emissivity versus model depth. Black line and left y-axis: gas temperature for each layer in the model. Colorful lines and right y-axis: cumulative of the emissivity contributions by each layer (weighted by length scale). By inspecting the A(V) ranges over which the cumulative emissivity curves rise the most, we can see the typical range of gas temperatures from which the lines originate. For the 1–1 S (7) line for example, the observed excitation temperature is ~2700 K, while around 75% of the emission originates from gas cooler than 1000 K, due to UV pumping.