Von kármán's constant

From Glossary of Meteorology

von Kármán's constant

(Or Kármán constant.) A constant k of the logarithmic wind profile in the surface layer.

It characterizes the dimensionless wind shear for statically neutral conditions,
where z is height, u* is friction velocity, and U is wind velocity. The Kármán constant was first determined in hydrodynamics. Presently a value of k = 0.40 ± 0.01 is assumed.
See logarithmic velocity profile.