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.

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.