Horizontal frictionless flow in which isobars and streamlines coincide, or equivalently, in which the tangential acceleration is everywhere zero. The balance of normal forces (pressure force, Coriolis force, centrifugal force) is then given by the gradient wind equation. Important special cases of gradient flow, in which two of the normal forces predominate over the third, are geostrophic flow, inertial flow, and cyclostrophic flow.