the clumping. Furthermore, when the doublet ratio is less than the ratio of the oscillator strengths, the inferred optical depth is always less than the actual optical depth. This means that derived parameters, such as the mass loss rate times the ionization fraction, $q$, is always under estimated. In their line-synthesis analyses Prinja [&]{} Massa (2010) reveal that the mean ratio of $\tau_{\rm{rad}}(w)$ of the blue to red Si[iv]{} components are rarely close to the canonical value of $\sim$ 2 (expected from atomic constants), and instead trend toward values closer to $\sim$1 and much less than 2. These results may be interpreted in terms of a photosphere that is partially obscured by optically thick structures in the outflowing gas.

We re-visit here the B supergiant sample of Prinja [&]{} Massa (2010) to measure the radial optical depths of the NACs separately for decoupled blue and red Si[iv]{} doublet components. We test the above notion that though the signs of clumping vary throughout the wind, the (blue-to-red) ratios of the optical depths near the NACs are closer to 2:1 (i.e. ratio of the oscillator strengths) and thus not so strongly affected by porosity effects.

![image](nacs_fig1.png)

NAC radial optical depths
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Our 