Geostrophic Winds
Geostrophic winds occur above the friction layer and develop in response to gradients in atmospheric pressure over large regions of the surface of the earth. Regions of high and low atmospheric pressure develop at the surface of the earth for a variety of reasons, including heating or cooling, and the rising or sinking of air due to surface topography or movements of air at higher levels in the troposphere. When pressure changes occur gradually over a large area, the isobars describing the horizontal change in pressure, or the pressure gradient, will take on a banded appearance, as seen below.
Fig. 1. Example of parallel isobars.
Much of the convection that takes place on earth is more localised. When low or high pressure develops more locally, the resulting high- and low-pressure regions are cell-like systems, and isobars describing the patterns of air pressure will include one or more closed, circular isobars, as shown below.
Fig. 2. Closed isobars around a low pressure cell.
The force acting on a parcel of air, causing it to accelerate from a region of relative high pressure to a region of relative low pressure is referred to as the pressure gradient force (or PGF). The PGF points from high pressure to low pressure, whether the regional isobars are parallel or concentric.
Fig. 3. Pressure gradient force and resultant winds on a non-rotating planet.
Winds on a non-rotating planet. The figure below 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).
Fig. 4. Hypothetical direction of winds on a non-rotating planet responding only to the pressure gradient force.
Winds on a rotating planet: the Coriolis effect. An air parcel initially at rest will move from high pressure to low pressure because of the pressure gradient force (PGF). However, because the earth is rotating, the winds are deflected by an apparent force called the Coriolis force, producing what is called the Coriolis effect. As an air parcel begins to move, it is deflected by the Coriolis force to the right in the northern hemisphere (to the left on the southern hemisphere).
Fig. 5. Direction of prevailing winds on a rotating earth influenced by the Coriolis effect.
Geostrophic winds. The horizontal air pressure gradient causes air parcels to accelerate across isobars from areas of high pressure toward areas of low pressure. The Coriolis effect then deflects air parcels to the right in the Northern Hemisphere. As the wind gains speed, the Coriolis effect increases in magnitude until it balances the pressure gradient force. The result is an unaccelerated horizontal wind blowing parallel to isobars that is called the geostrophic wind.
