A hypothetical layer of air at the bottom of a statically neutral atmosphere surrounding a rotating planet, where surface friction and small eddies (local turbulence) cause ageostrophic, cross-isobaric winds. The spiral shape of a hodograph of this flow is called an Ekman spiral. The Ekman layer is not observed in the earth's atmosphere because of the existence of large eddies (nonlocal turbulence) creating a mixed layer, and because the earth's troposphere is statically stable on average, causing a thermodynamic cap to the boundary layer rather than a dynamic cap at a theoretical Ekman layer depth of 20 u*/f, where u* is the friction velocity and f is the Coriolis parameter. Compare atmospheric boundary layer.