Wiktionary
geostrophic wind level
n. (context meteorology English) The lowest level at which the wind becomes geostrophic. In practice, the geostrophic wind level is between 1.2 kilometers (3,928 feet) and 1.6 kilometers (5,238 feet). This wind level probably 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 planetary boundary layer, that is, the base of the free atmosphere.