With a geostrophic wind blowing along straight, parallel isobars, the pressure gradient force is exactly balanced by coriolis force. When the isobars are curved around a centre of high or low pressure centrifugal force (acting out from the centre of curvature) is introduced and the gradient wind is the geostrophic wind modified by centrifugal force. With a centre of high pressure the centrifugal force is acting in the same direction as pressure gradient force; effectively increasing the pressure gradient force and causing the gradient wind to be “hi round a hi” (stronger than the geostrophic wind). With a centre of low pressure the centrifugal force is opposing the pressure gradient force; effectively reducing the pressure gradient force and causing the gradient wind to be “lo round a lo” (weaker than the geostrophic wind).
Difference between Gradient and Actual Wind
It pays to remember that if the earth didn’t rotate, the wind would blow straight from an area of high pressure, into an area of low pressure, however because of the earth’s rotation, the coriolis force acts on this airflow as it attempts to move from high to low pressure.
In a high pressure system, we have sinking air, which as it hits the surface has nowhere to go but away from the centre of the system. The coriolis force then acts on this air to produce a deflection to the left (in the southern hemisphere).
The opposite occurs in a low pressure system. Air is rising therefore air is being sucked into the system at the surface. However once again due to coriolis force it takes a deflection to the left as it attempts to move over the surface of the earth. This is why we get clockwise or cyclonic flow (cyclonic simply meaning “as the earth rotates”) around a low pressure system, and anti-clockwise or anti-cyclonic flow around a high pressure system (once again in the southern hemisphere).
The other thing to be aware of is that the coriolis force is the strongest when the wind speed is highest, and zero when there is no wind.
Gradient wind is accepted by the met man to be above 3000 feet. From this height up the wind flow follows parallel to the isobars as you said. Below 3000 feet the wind speed is slowed down due to friction created by the surface of the earth, and we call this the surface wind. Now When we slow the wind speed down we reduce the coriolis force, and this no longer counters the pressure gradient force, so now the surface wind deflects across the isobars in favour of the pressure gradient force. The amount by which this deflects depends upon the nature of the surface. It is the surface wind (below 3000 feet) which is deflected to the right of the gradient wind (above 3000 feet) no matter whether in a high or low pressure system.
This degree of deflection is accepted to 30 degree’s over land ( due to more friction over land therefore reducing the coriolis force) and 10 degrees over the sea due to less friction over ocean. The point to remember is that the surface wind is always deflected to the right of the gradient wind, or if you remember your backing and veering, the surface wind has veered with respect to the gradient wind.