Development of Geostrophic Winds

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• 00:00 Introduction to…
• 2:00 Coriolis Force
• 3:50 Geostrophic Wind
• 4:48 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

The dominant forces of pressure gradient force and Coriolis force combine to form geostrophic wind flows. In this lesson, we'll examine why many of the winds in the atmosphere actually blow parallel to the prevailing isobars.

Introduction to Geostrophic Wind

There are a number of forces that can either change the force or direction of wind. Two of the biggest forces of wind vectors are the pressure gradient force and the Coriolis force. When these two major forces are combined and are equal to each other we get a type of wind called a geostrophic wind. In this lesson, we'll study how pressure gradient force and Coriolis force are created and the formation of geostrophic winds.

One of the largest such drivers is the pressure gradient force. The pressure gradient force is the force that drives air from high to low pressure. Systems in nature are always trying to stay at the lowest energy state possible. High pressure systems are high energy states. If there is a nearby lower pressure area, air will freely move from the high pressure to low pressure area in an attempt to equalize this energy gradient.

You may have been introduced to the idea of isobars, which are imaginary lines of equal pressure. Because the air is wanting to move as quickly and efficiently from high to low pressure it will move the shortest distance between these areas, which is always perpendicular to the isobars. This is described in the following equation for the horizontal portion of the pressure gradient force:

P= inverse -1/p * p

The pressure gradient force, here shown as P, is equal to the inverse of the density times the pressure gradient. The inverse sign is an upside down triangle. The negative (-) at the beginning of the equation designates that we move from high to low across the pressure gradient.

Now, if this was the only force acting on the atmosphere, we could easily predict wind direction. Wind should move from high to low pressure perpendicular to the isobars; however, we observe that higher in the atmosphere, wind is actually moving parallel to the isobars. How can that happen?

Coriolis Effect

We must look at some other forces that act on wind. Another dominant force on wind direction is the Coriolis effect. The Coriolis effect is the diversion of the path of air due to the rotation of the Earth. The Earth is spinning from west to east. The Earth is a sphere, and therefore all points are traveling with the same angular velocity. However, because a point at the equator must travel a much further distance than a point near the pole for full rotation, areas near the pole are actually traveling at a much higher linear velocity.

When an object moves either closer or further from the equator its original momentum is preserved, giving the path a diversion off its original course. Paths in the Northern hemisphere are drug to the right, and paths in the Southern hemisphere are drug to the left. So, if we have wind that is originally blowing according to the pressure gradient force, this wind will be deflected by the Coriolis force. Now, the Coriolis force is not present at the equator, but it increases in intensity the further you approach the poles. The increase in force is due to the greater divergence in linear speed observed at the equator.

Coriolis force is described by this equation:

Æ’c = 2 * omega * sin * Phi

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