From AMS Glossary
A layer of order 1–10 m in thickness adjacent to the surface, where influences of stratification and Coriolis force are negligible, but far enough away from the surface that viscous and individual roughness-element effects are negligible.
Namely, it is where
, where z is height above the surface and L is Obukhov length, the depth of the sublayer.
Brutsaert, W. 1982. Evaporation into the Atmosphere. Kluwer, . 57–64.