Comparison between a Π → Δ (red, T_rot = 5.24 K), Σ → Σ (dashed black, T_rot = 2.17 K) and Σ → Π (dash-dotted blue, T_rot = 4.59 K) transition. The rotational temperatures are matched, so that the models have the same line ratio of the two strongest transitions. In this temperature range, the Π → Δ and Σ → Σ models have the same line ratios, but different temperatures. The Σ → Π model has a stronger transition at lower wave numbers, which is as strong as the weak transition at higher wave numbers.