Aerodynamically smooth surface

From Glossary of Meteorology

aerodynamically smooth surface

A surface with irregularities sufficiently small to be entirely embedded in the laminar sublayer.

Thus, the smoothness of a surface will depend on the Reynolds number. The irregularities are taken to be sufficiently dense and similar so that only the average height need be specified. A surface is aerodynamically smooth if
where u* is friction velocity, z0 is aerodynamic roughness length, and ν is the kinematic viscosity of air. The critical Reynolds number is in the range of 2.5 ≤ Ref ≤ 70.
Compare aerodynamically rough surface.