the wind, so we can never be certain whether we are seeing a part of the wind with a covering factor of 1 (the doublet ratios verify this). On the other hand, NACs, almost by definition, are parts of the line profiles with the largest optical depth. They are formed in a region of the wind where the velocity law flattens out as it approaches the terminal velocity. As a result, all structure at radii greater than the region where the velocity is $\sim$ 0.9$\times$ terminal velocity ($v_\infty$) contribute to a very small part of velocity space. Consequently, the wind becomes “smooth” on the Sobolev length at high velocity, so column densities are uniformly distributed over the stellar disk. This requires the structures contained within a Sobolev length to cover the star, but the chances are high for that since the Sobolev length is so large. Consequently, if the covering factor is unity anywhere, it is near terminal velocity, hence the NACs.

In their study demonstrating wide-spread clumping in B supergiant winds, Prinja [&]{} Massa (2010) exploited Si[iv]{} 1400 resonance line doublets of B0 to B5 supergiants where the red and blue components of the doublet are decoupled and formed 