Processing the data from a large variety of zero-pressure-gradient boundary layer flows shows that the Reynolds-number-dependent scaling law, which the present authors obtained earlier for pipes, gives an accurate description of the velocity distribution in a self-similar intermediate region adjacent to the viscous sublayer next to the wall. The appropriate length scale that enters the definition of the boundary layer Reynolds number is found for all the flows under investigation.

Another intermediate self-similar region between the free stream and the first intermediate region is found under conditions of weak free-stream turbulence. The effects of turbulence in the free stream and of wall roughness are assessed, and conclusions are drawn.