of CIRs has been studied hydrodynamically in 2-D by Cranmer & Owocki, (1996) and Lobel & Blomme (2008), and in 3-D by Dessart (2004). The 2-D calculations also modelled the effect of the CIRs on the emergent line profiles, showing that they can reproduce the typical migrating behaviour of the DACs. They find that the optical depth enhancements are essentially caused by a [*velocity*]{} kink discontinuity slowly migrating outward in a fixed stellar frame.

More pertinent to the present study is that the spectroscopic end-products of the evolution of either CIRs or large scale clumps, are localised narrow absorption components (NACs), which are commonly seen in single (snapshot) UV spectrograms (e.g. Lamers et al. 1982; Howarth [&]{} Prinja 1989). The definition of a NACs adopted here is that of narrow absorption components that occur near the terminal velocity.  While their strengths may change with time, they tend to remain fixed is velocity.  Our study addresses the information that can be determined from the NACs, while  acknowledging that DAC formation and propagation is far more complex (e.g. Brown et al. 2004) and beyond the scope of the current work. The surveys cited above suggest that NACs may be present, at some 