winds balanced by the Coriolis and Pressure Gradient forces
An air parcel initially at rest will move from high pressure to low
pressure because of the
pressure gradient force (PGF).
However, as that 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).
As the wind gains speed, the deflection increases until the Coriolis force
equals the pressure gradient force. At this point, the wind will be blowing
parallel to the isobars. When this happens,
the wind is referred to as geostrophic.
The movie below illustrates the process mentioned above, while the
diagram at right shows the two forces balancing to produce the
geostrophic wind. Winds in nature are rarely exactly geostrophic,
but to a good approximation, the winds in the upper troposphere
can be close. This is because winds are only considered truly
geostrophic when the isobars are straight and there are no other
forces acting on it -- and these conditions just aren't found too often