geostrophic wind

In atmospheric science, geostrophic flow (/ˌdʒiːəˈstrɒfɪk, ˌdʒiːoʊ-, -ˈstroʊ-/) is the theoretical wind that would result from an exact balance between the Coriolis force and the pressure gradient force. This condition is called geostrophic equilibrium or geostrophic balance (also known as geostrophy). The geostrophic wind is directed parallel to isobars (lines of constant pressure at a given height). This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there were no friction (e.g. above the atmospheric boundary layer) and the isobars were perfectly straight. The geostrophic wind merely reflects the horizontal pressure gradient as the driving force of atmospheric motion, while the actual near-surface wind requires the synergistic modulation of multiple factors including the frictional force, vertical atmospheric stratification and thermal wind. As a result, the ratio between actual wind speed and geostrophic wind speed, and the wind direction angle difference between the actual wind and the geostrophic wind vary significantly with environmental conditions. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation. Geostrophic flow in air or water is a zero-frequency inertial wave.

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