The depth below the ocean surface to which the stress (horizontal force/unit area) of the wind reaches. To measure this depth, the turbulent stress must be directly measured. This is rarely done, but is more often inferred through observations of current profiles and fitting to an Ekman layer profile (e.g., Eqs. (9. 9) and (9. 10) in Pond and Pickard (1978)). Because of the strong effect stratification has on turbulent transport, the depth of frictional influence is also taken to be equal to the depth of the mixed layer, and this is in turn found by observations of temperature and/or salinity profiles.