Relative populations of the OH levels. E_u is the energy of the upper level (units of 10³ cm^-1, equivalent to 1.99 × 10^-13 erg) and g_u is its statical weight. The solid lines are the values expected for a thermal population with a vibrational temperature T_vib = 9000 K and a rotational temperature T_rot = 180 K. The dashed lines include a given fraction of hot molecules with T_rot = T_vib. The upper-left panel shows a region of the spectrum with some of the high-excitation lines from non-thermal levels.