Ekman layer

(Sometimes called spiral layer.) 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.

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