Clockwise from top-left: modelled excitation temperatures of the 3.264 GHz (dark blue), 3.349 GHz (dark orange), and 3.335 GHz (red) ground-state transitions of CH as a function of CH column densities for the physical conditions derived for the different velocity components towards Sgr B2 (M), G34.26+0.15, W51 E, and W49 (N). The differing line styles are used to indicate results for the different velocity components as labelled. The inset panels expand on the T_ex values for CH column densities between 3 × 10¹³ and 10¹⁵ cm⁻² and the greyshaded regions highlight the column density intervals relevant to the velocity components in each respective source.