Development of geostrophic wind flowing along parallel isobars.

The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis effect and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars. 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 and the isobars were perfectly straight. 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.

Origin

Air moves from areas of high pressure to areas of low pressure, due to the pressure gradient force. As soon as the air starts to move, however, the Coriolis “force” deflects it. The deflection is to the right in the northern hemisphere, and to the left in the southern hemisphere. As the air moves from the high pressure area, its speed increases, and Coriolis deflection also increases. The deflection increases until the Coriolis and pressure gradient forces are in geostrophic balance: at this point, the air flow is no longer moving from high to low pressure, but instead moves along an isobar. The geostrophic balance helps to explain why, in the northern hemisphere, low pressure systems (or cyclones) spin counterclockwise and high pressure systems (or anticyclones) spin clockwise, and the opposite in the southern hemisphere.

Winds on a Non-Rotating Planet 

Hypothetical direction of winds on a non-rotating planet responding only to the pressure gradient force

Direction of prevailing winds on a rotating earth influenced by the Coriolis effect.

The figure here illustrates how winds would develop in the tropics on Earth if it did not rotate. On a non-rotating planet, the principle force which would act on a parcel of air between the subtropical high and the equatorial low would be the pressure gradient force. If that force were the only force present, and friction were not acting on the parcel, the air parcel would accelerate in the direction of this force, toward the equator. On such a planet, the wind would flow in a direction perpendicular to the isobars, the precise direction of the pressure gradient force. High and Low pressure areas on a non-rotating planet would quickly fill in as air motions are principally from the regions of “excess air” (the high pressure centers) to regions of air deficit (the low pressure center).

Geostrophic currents

Flow of ocean water is also largely geostrophic. 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. Satellite altimeters are also used to measure sea surface height anomaly, which permits a calculation of the geostrophic current at the surface.

Source(s): Wikipedia

Geostrophic Winds

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