Velocity-integrated intensities of the J = 1−0 (top) and J = 2−1 (bottom) lines of the main CO isotopologs as a function of H₂ column density. The data are color-coded by cloud: blue circles for California, green for Perseus, and red for Orion A. The dashed line in the ¹³CO(2−1) panel indicates the slope of a linear trend. No J = 2−1 data were taken for the California cloud apart from 13 C¹⁸O spectra from high column density positions. The lowest value of the intensity scale (0.1 K km s⁻¹) approximately corresponds to the line detection limit.