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What cause the Coriolis effect to occur? Why can't the winds move in exactly the direction specified by the pressure gradient?


The coriolis acceleration occurs when a particle is in motion over a rotating surface, such as the turning Earth. This is complicated by the Earth being a turning sphere. The rate of acceleration is proportional to the speed of the particle and depends on the rotational speed of a plane through the latitude where you are. The further north you are in the Northern Hemisphere, the lower the rotational speed of that plane and thus the smaller the coriolis acceleration.

The pressure-gradient force is the force that makes particles move from high to low atmospheric pressure. If there were no other forces at work, the wind would move from high to low pressure, perpendicular to the isobars.

The coriolis acceleration makes particles move towards the right in the Northern Hemisphere, in other words from low to high pressure. Therefore the coriolis acceleration would balance the pressure gradient force (if the Earth's surface had no friction). This is referred to as the geostrophic balance and the resulting wind would flow along the isobars. That wind is called the geostrophic wind.

However, the friction of the Earth's surface slows the air movement and reduces the effectiveness of the coriolis acceleration, thereby causing the wind to flow slightly across the isobars from high to low pressure. This results in air movement into a low pressure system (causing it to decrease in pressure) and out of a high pressure system (causing it to expand and decrease in pressure).

So, you can see how important the various forces (pressure-gradient, coriolis, frictional) are in producing the wind.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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