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We seek to establish evidence in UV P Cygni line profiles that the signs ofwind clumping and porosity vary with velocity. We aim to demonstrateempirically that while at most wind velocities optically thick clumps coveronly a fraction of the stellar surface, close to the terminal velocity wherenarrow absorption components (NACs) appear in UV lines the covering factor isapproximately unity. SEI line-synthesis models are used to determine the radialoptical depths of blue and red components of the SiIV 1400 resonance linedoublet in a sample of 12 B0 to B4 supergiants. We focus on stars with welldeveloped NACs and relatively low terminal velocity so that the SiIV doubletcomponents can be treated as radiatively decoupled and formed independently.For all 12 stars the mean optical depth ratio of the blue to red components iscloser to ~ 2 (i.e. the ratio of oscillator strengths) in the NACs than atintermediate and lower velocities. The product of mass-loss rate and Si^3+ ionfraction calculated from the NAC optical depths is a factor of ~ 2 to 9 highercompared to mass-loss values sampled at ~ 0.4 to 0.6 of the terminal velocity.Since the wind effectively becomes `smooth' at the high NAC velocities and thecolumn density is uniformly distributed over the stellar disk, the opticaldepths of the NACs are not seriously affected by porosity and this feature thusprovides the most reliable measurement of mass-loss rate in the UV lines.Applications of this result to the weak-wind problem of late O-dwarf stars andthe "PV mass loss discordance" in early O supergiants are discussed.

UV diagnostic of porosity-free mass-loss estimates in B stars