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# The Coriolis Effect: Earth's Rotation and Its Effect on Weather

The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around the Earth.

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### Subjects

Earth Science, Meteorology, Geography, Physical Geography, Physics

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Imagine you have amazing strength. You can throw a ball like Superman. You are standing at the Equator. The Equator is an imaginary line around the middle of Earth. You want to throw a ball to a friend, who is standing somewhere in North America. What's going to happen?

If you try throwing the ball straight to your friend, things won't go as planned. The ball will land slightly to your friend's right. The reason for this is the Coriolis effect.

The Coriolis effect is caused by the planet's rotation. Earth is constantly rotating, or spinning. It is spinning from west to east. This is left to right on a map. Every 24 hours, Earth makes a full spin. Not every point on Earth is moving at the same speed, though. Near the Equator, points are rotating faster than at the poles.

The Equator divides the planet into two halves. The northern half is called the Northern Hemisphere. The southern half is called the Southern Hemisphere. Earth is widest at the Equator. In order to make a full spin in 24 hours, points at the Equator have to go a longer distance. Points near the poles have to go a much shorter distance. That means points at the Equator are moving much faster than points near the poles. Regions near the Equator move at almost 1,600 kilometers (1,000 miles) an hour. Near the poles, Earth moves extremely slowly.

This is why the ball did not reach your friend in North America. At the Equator, you and the ball are moving east. Your friend is also moving east, but at a slower speed. When you throw the ball, it will move toward your friend at first. But it will also move east at a faster speed than him. So it will land to your friend's right.

Now let's pretend you're standing at the North Pole. When you throw the ball to your friend, it will again land to his right. This time, it's because he's moving faster than you; he has moved ahead of the ball.

In real life, the Coriolis effect has a large effect on the weather. It changes how winds blow. It makes winds bend to the right in the Northern Hemisphere. In the Southern Hemisphere, it makes them bend left. It also causes cyclones. These are large, rotating masses of air. In the Northern Hemisphere, cyclones rotate counterclockwise. In the Southern Hemisphere, they rotate clockwise.

The Coriolis Effect Closer to Home

Here's one last example of the Coriolis effect. It's something you can try yourself. Suppose you and a friend are throwing a ball back and forth. Imagine you're sitting on a merry-go-round. When the merry-go-round is still, playing catch is easy. Things are different when the merry-go-round is rotating. The ball won't reach your friend unless you throw it extra hard. If you throw it normally, the ball will curve to the right.

The ball is actually flying in a straight line. It is you and your friend who are moving out of the way. You are spinning because of the merry-go-round.

Fast Fact

Coriolis Force
The invisible force that appears to deflect the wind is the Coriolis force. The Coriolis force applies to movement on rotating objects. It is determined by the mass of the object and the object's rate of rotation. The Coriolis force is perpendicular to the object's axis. The Earth spins on its axis from west to east. The Coriolis force, therefore, acts in a north-south direction. The Coriolis force is zero at the Equator.

Though the Coriolis force is useful in mathematical equations, there is actually no physical force involved. Instead, it is just the ground moving at a different speed than an object in the air.

Fast Fact

Polar PowerThe Coriolis force is strongest near the poles, and absent at the Equator. Cyclones need the Coriolis force in order to circulate. For this reasons, hurricanes almost never occur in equatorial regions, and never cross the Equator itself.

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###### Editor
Jeannie Evers, Emdash Editing, Emdash Editing
###### Producer
National Geographic Society

October 19, 2023