The lowest level at which the wind becomes geostrophic in the theory of the Ekman spiral, proportional to (ν/sinφ)½, where ν is the kinetic eddy viscosity and φ the latitude. In practice it is observed that the geostrophic wind level is often between 1 and 2 km, and it is assumed that this marks the upper limit of frictional influence of the earth's surface. The geostrophic wind level may be considered to be the top of the Ekman layer and planetary boundary layer, that is, the base of the free atmosphere.