Column density ratios predicted by standard TDR models, for a turbulent rate-of-strain of 3 × 10^-11 s^-1; ion-neutral velocity drifts of 4 (blue), 6 (magenta), and 8 (red) km s^-1; and densities of 30 (triangles), 50 (squares), 100 (circles), and 300 (upside down triangles) cm^-3. The column densities are computed for a total hydrogen column density N_H of 1.8 × 10²¹ cm^-2, assuming an homogeneous shielding A_V over the entire line-of-sight. Along each curve, A_V varies between 0.1 and 1.0 from left to right. Grey shaded regions indicate the observed range of values.