Correlation of integrated intensities of H i (left) and CO(2–1) (right) with [C ii] integrated intensities for BCLMP302 (black squares), central region (red triangles), and BCLMP691 (blue empty squares; Braine et al. 2012) and for positions along the major axis of M 33 measured with ISO/LWS at 280 pc resolution (green filled circles; Kramer et al. 2013). For the central and BCLMP302 region, [C ii], CO(2–1) and H i are integrated only over the range of velocities in which [C ii] is detected at the individual positions. The dashed curved lines in the left panel show the predicted I([C ii]) for low-density diffuse atomic H i clouds as a function of I(H i), corresponding to the pressures P = 2000 and 8000 K cm^-3 for a kinetic temperature of 100 K. The box in dashed lines shows the region occupied by the [C ii] emitting H i clouds analyzed by the GOT C⁺ project (Langer et al. 2014). Both panels show lines (blue continuous) of proportionality to guide the eye.