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The Coriolis Effect

Understanding Winds on a Rotating Earth


The Coriolis Effect is an often difficult weather phenomena to discuss and understand. Because the Coriolis effect is an apparent motion (dependent on the position of the observer), many cannot visualize the effect on planetary scale winds. Through this tutorial, you will gain an understanding of the reason winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The History

To begin, the Coriolis effect was named after Gaspard Gustave de Coriolis who first described the phenomenon in 1835.

Born in 1792 in Paris, Gustave de Coriolis was a pioneering mathematician and respected teacher. Although his descriptions of mathematics are difficult to follow, explanations of the physics principles involved in Earth rotation are available for the curious.

Winds blow as a result of a difference in pressure. This is known as the pressure gradient force. Think of it this way. If you squeeze a balloon at one end, the air automatically follows the path of least resistance and works towards an area of lower pressure. Release your grip and the air flows back to the area you (previously) squeezed. Air works in much the same way.

The total forces applied to the winds are generated by high and low pressure systems. In fact, this is the same phenomenon that changes wind speed. The greater the difference between two areas of pressure, the higher the wind speed.

Use Your Imagination...

Now, let’s imagine you are far away from the earth and you are observing a storm moving towards an area. Since you are not connected to the ground in any way, you are observing the rotation of the earth as an outsider. You see everything moving as a system as the earth travels around at a speed of approximately 1070 mph (1670 km/hr) at the equator. You would notice no change in the direction of the storm. The storm would appear to travel in a straight line.

However, on the ground, you are traveling at the same speed as the earth and you are going to see the storm from another perspective. This is due largely to the fact that the rotational speed of the earth depends on your latitude. To find the rotational speed where you live, take the cosine of your latitude, and multiply it by the speed at the equator, or go to the Ask an Astrophysicist site for a more detailed explanation. For our purposes, you basically need to know that objects on the equator travel faster and farther in a day than objects at higher or lower latitudes.

Now, imagine you hovering exactly over the North Pole in space. The rotation of the earth, as seen from the vantage point of the North Pole, is counterclockwise. If you were to throw a ball to an observer at a latitude of about 60degrees North on a non-rotating earth, the ball would travel in a straight line to be caught by a friend. However, since the earth is rotating underneath of you, the ball you threw would miss your target because the earth is rotating your friend away from you! Keep in mind, the ball is STILL traveling in a straight line! But the force of rotation would make it appear that the ball is being deflected to the right.

The Southern Hemisphere

The opposite is true in the Southern Hemisphere. Imagine standing at the South Pole and seeing the rotation of the earth. The earth would appear to rotate in a clockwise direction. If you don’t believe it, try taking a ball and spinning it on a string.
  1. Attach a small ball to a string of about 2 feet in length.
  2. Spin the ball counterclockwise above your head and look up.
  3. Although you are spinning the ball counterclockwise and DID NOT change direction, by looking up at the ball it appears to be going clockwise from the center point!
  4. Repeat the process by looking down at the ball. Notice the change?
In fact, spin direction does not change, but it appears to have changed. In the southern Hemisphere, the observer throwing a ball to a friend would see the ball being deflected to the left. Again, remember that the ball is in fact traveling in a straight line.

If we use the same example again, imagine now that your friend has moved farther away. Since the earth is roughly spherical, the equatorial region must travel a greater distance in the same 24 hour period than an area of higher latitude. The speed, then, of the equatorial region is greater.

Coriolis Videos

As winds are forced from high and low pressure belts at 0, 30, 60, and 90 degrees, the direction of deflection is always to the right in the Northern Hemisphere and to the Left in the Southern Hemisphere. But don’t just take my word for it! If you are at all confused, animations and free videos on the Coriolis Effect will help to clear up any misunderstandings about the phenomenon.

In addition, several key lesson plans on the Coriolis Effect are available for teachers. Each lesson links to high quality animations, images, and photo documentation to make the hand-on science lessons of Coriolis a reality for students.

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