$\sim$ 0.3 $v_\infty$). The velocity ranges adopted for the individual NACs are listed in Table 3, and in most cases the NACs are measured in at least 3 velocity bins; nevertheless the standard deviation in $\tau_{\rm{rad}}(w)$ can be quite large. The NAC blue-to-red velocity averaged optical depth ratio, $\langle \tau_{\rm
{rad}}^{\rm blue}/\tau_{\rm {rad}}^{\rm red}\rangle_w$, is also given in Table 3 where it is compared to the corresponding ratio measured at lower velocity ($\sim$ 0.4 $-$ 0.6 $v_\infty$). In all 12 B supergiant stars we note that $\langle \tau_{\rm {rad}}^{\rm blue}/\tau_{\rm {rad}}^{\rm red}
\rangle_w$ is higher (and closer to $\sim$ 2.0) in the NAC than at lower velocities (see Fig. 2). The difference in $\langle \tau_{\rm {rad}}^{\rm
blue}/\tau_{\rm {rad}}^{\rm red} \rangle_w$ between the NACs and low velocity wind demonstrates two key points; (i) the signs of clumping vary as a function of velocity throughout the wind and that the stellar disk covering factor is relatively low for most of the outflow, and (ii) the ratio of optical depths in the NACs is closer to $\sim$ 2:1 i.e. the ratio of $f$-value. The covering factor is thus $\sim$ 1 at the (high) NAC velocity. [*This suggests that the optical depths of the 