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Coriolis force is the earth's affect on "bending" winds. Because the earth rotates, winds don't move straight-line, they curve and bend.

Therefore, wind is pressure equalizing itself in the atmosphere, while the earth's rotation bends this equalization of pressure.

The geostrophic wind can be taken to represent the true wind.The coriolis force is an apparent force created due to the rotation of the earth.It is maximum at the poles and zero at the equator.It deflects the wind to the right in the northern hemisphere and to the left in the southern hemisphere.The hurricanes get the rotating motion because of this apparent force only.

Usually a parcel of air is acted by three forces.They are pressure gradient force, coriolis force and centrifugal force due to the curvature of the isobars(isobars are lines connecting the places having same pressure on a weather map).If these isobars are straight lines(without curves), then the first two forces only will act and the centrifugal force will be absent.

Now we shall consider the case where the isobars are straight lines parallel to each other.In this case, the pressure gradient force and the coriolis force only will act on the air parcel.The pressure gradient will act from high pressure to low pressure and hence will be perpendicular to isobars.But the coriolis force will try to deflect the wind which is blowing perpendicular to the isobars to the right side.So ,the wind which is blowing from the high pressure to the low pressure and perpendicular to isobars will slowly turn to the right due to coriolis force and become parallel to the isobars.This parallel wind to the isobars is called geostrophic wind and represents the true wind.

So,a wind which blows under the balance of force due to pressure gradient force and coriolis force is called geostrophic wind.

'Under the balance' means the wind blows under the influence of both the forces.

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 (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 or the centrifugal force from curved fluid flow. 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.