Geostrophic Balance

For the coriolis force to balance the pressure gradient force, the wind must blow parallel to the coast, with high pressure to the right and low pressure to the left. That way, the horizontal pressure gradient is trying to accelerate the air to the left, and the coriolis force is trying to accelerate the air to the right. At just the right wind speed, the two forces cancel and the air blows steadily.

This is really weird, folks! It made sense when the wind blew from high to low pressure, but now we're talking about air blowing at right angles to the pressure gradient! Strange, but true. This is what happens within the larger-scale weather systems that last for more than several hours. The wind blows at right angles to the pressure gradient, with high pressure to its right and low pressure to its left, and its speed is proportional to the magnitude of the pressure gradient. Only by doing this can the forces balance and the air keep its acceleration to a minimum. This balance of forces is called geostrophic balance, and the wind speed and direction necessary to attain geostrophic balance is called the geostrophic wind.

How do you estimate this wind on a map? Actually, it's simple. Consider the surface pressure map below. The black lines are the isobars, or lines of constant pressure.

If the wind is to blow at right angles to the pressure gradient, it must blow in a direction such that pressure neither decreases or increases. Thus, it follows the isobars, the lines of constant pressure on a weather map! As for the speed, you should know that the closer together the isobars, the stronger the pressure gradient. Therefore, the geostrophic wind speed is stronger where the isobars are closer together.

Now think about direction. If high pressure must always be to the right, then air must circulate clockwise around a high pressure center and counterclockwise around a low pressure center. Check it out on the map and you'll see how it works. On TV, they always tell you that the wind circulates counterclockwise around a low pressure center, but they never tell you why. Well, this is why.

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