  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 of distances
from the wall adjacent to the viscous sublayer. 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.
