The Coriolis Effect

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

Simple Illustrations

Imagine you and a friend are riding on a merry-go round while playing catch. You throw the ball to your friend:

Only to see it mysteriously travel a curving path.

To someone watching all this from outside, like the nice people coming to escort you and your friend out of the amusement park, there's no mystery at all. The ball does travel a straight line, but you and your friend are moving as well.

Because the earth is spherical, objects near the equator rotate faster than objects near the pole. Imagine we launch a rocket from Quito, Ecuador (red), on the equator due north toward Miami (purple). We launch another one due south from Toronto (green).
Quito is rotating east at 493 m/sec, Miami (26 N) is rotating at 443 m/sec. The rocket from Quito is moving faster than Miami and misses it to the east.  Toronto (44 N) is moving at 355 m/sec, slower than Miami, and misses it to the west. Note that both rockets are deflected to the right as seen by someone on the ground.
Here's an animation showing what happens.

Those illustrations help illustrate why winds and ocean currents curve as they blow across the earth's surface, but they leave some puzzling questions. If something moves and is continuously deflected by the Coriolis Effect, eventually it should end up moving east or west. Since it's moving over a region where everything is moving at the same speed, it shouldn't be deflected any more, right? Instead, anything moving on the earth is deflected by the Coriolis Effect, even if it's moving east or west. We need a more sophisticated illustration.

The Next Level

Imagine you're flying in a spaceship in a straight line past the earth and pretty much passing over the center of the United States. As you fly along, a funny thing happens. The United States twists to your left.

The reason, of course, is that the earth is rotating and from where you sit the United states is following a curved path (actually an ellipse). In the course of a few hours the United States rotates appreciably.

From the perspective of someone on the ground plotting the spaceship's movements on a map, the spaceship appears to turn to the right. It also drifts north and south because the spaceship travels in a straight line whereas the United States rotates in an arc beneath it.

If the spaceship flies over Australia instead, Australia appears to turn to the right.

Over the course of a few hours, Australia rotates significantly.

To a perplexed wombat watching from the ground, of course, the spaceship appears to twist to its left.

Below is a general view of the effect. We start off from a central location in red and launch missiles at the four points shown in blue and green. We're looking down on the earth from space. Note that the lines of flight are not north-south and east-west from the vantage point of someone on the ground.

As the missiles travel, note that the missiles end up apparently deflected to the right of their aiming points, even though from a vantage point in space they are traveling straight lines. Their paths on the ground curve because the earth rotates beneath them.

The animation above is for the Northern Hemisphere. In the Southern Hemisphere the apparent deflection is to the left.

Keeping It Straight

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Created 15 January 2007, Last Update 31 May 2020