et al. (2012) have presented theoretical models for specific wind structure geometry which demonstrate how this effect can occur. In this paper, we have attempted to quantify exactly how large the effects of porosity on optical depths can be.

By considering the decoupled Si[iv]{} $\lambda\lambda$1400 blue and red doublet components of B supergiants with relatively low terminal velocity (i.e. $v_\infty$/Si[iv]{} $\Delta_{\rm sep}$ $\simlt$ 0.65), we have demonstrated that the blue-to-red optical depth ratios in NACs are closer to $\sim$ 2 (as expected from atomic constants) than at lower wind velocities. The results indicate that while at most of the wind velocities optically thick clumps cover only a fraction of the stellar surface, in the near-$v_\infty$ NAC region the covering factor is $\sim$ unity. This implies that the NACs provide a better empirical UV measure of mass-loss rates that are not seriously affected by porosity.

![The blue-to-red radial optical depth ratio in the NACs plotted against the corresponding ratio measured at low wind velocity ($\sim$ 0.4$-$0.6 $v_\infty$). Stars with $v_\infty$/Si[iv]{} $\Delta_{\rm sep}$ $\simlt$ 0.5 and $\simlt$ 0.65 are plotted in black closed and red open circles, respectively. ](nacs_fig2.png)

The SEI model fitting parameters in Table 2 were used to calculate the 