Over a city or rough terrain, the wind gradient effect could cause a reduction of 40 % to 50 % of the geostrophic wind speed aloft; while over open water or ice, the reduction may be only 20 % to 30 %.
22.
Just as multiple weather balloons that measure pressure as a function of height in the atmosphere are used to map the atmospheric pressure field and infer the geostrophic wind, measurements of density as a function of depth in the ocean are used to infer geostrophic currents.