Comparison of H I–H₂ transition theory predictions with observations. Top panel: derived probability distribution function of αG from excitation of C I fine-structure levels. The green shaded region shows the interval estimate on αG corresponding to 0.683 confidence level. Bottom panel: theoretical H I surface density of a single H I–H₂ transition layer as a function of αG Eq. (5) for shown by a solid blue line. The two dashed blue lines correspond to the derived range for . The observed total H I column density is shown as a red horizontal line with downwards arrows, since part of the H I is likely not to be associated with the molecular cloud.