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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.

3 - 12+

### Subjects

Earth Science, Meteorology, Geography, Physical Geography, Physics

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Imagine you have superhuman strength. You are standing at the Equator, which is the 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 our planet's rotation. Earth is constantly rotating, or spinning, from west to east. Every 24 hours, Earth makes a full rotation. Different points on Earth move at different speeds, though. Points near the Equator rotate faster than points near the poles.

The Equator divides the planet into two halves. These halves are called the Northern and Southern Hemispheres. Earth is wider at the Equator. So points along the Equator have to cover a longer distance in order to make a full rotation in 24 hours. These points move at nearly 1,600 kilometers (1,000 miles) an hour. Near the poles, however, things are very different. Earth is rotating extremely slowly there.

This explains why the ball did not reach your friend. At the Equator, you and the ball are already moving east at a certain speed. In North America, your friend is moving east more slowly. When you try throwing the ball, the ball is moving toward your friend at first. But it's also moving east more quickly than your friend is. So it will land to your friend's right.

Now let's pretend you're standing at the Pole">North Pole instead. Your friend is still waiting in North America. When you throw the ball to your friend, it will again land to your friend's right. This time, it's because he's moving faster than you are; he has moved ahead of the ball. No matter where you are in the Northern Hemisphere, the ball will always move east, or to the right.

In real life, the Coriolis effect has a large impact on the weather. In the Northern Hemisphere, it makes air currents bend to the right. In the Southern Hemisphere, it makes currents bend left.

Weather Patterns

Cyclones are shaped by the Coriolis effect. Cyclones are large air masses that rotate around a center. As they rotate, cyclones pull air into their center, or "eye." These air currents are pulled in from all directions. In the Northern Hemisphere, they bend to the right. This makes the cyclone rotate counterclockwise. In the Southern Hemisphere, currents bend to the left. This makes cyclones rotate clockwise.

The Coriolis effect also has an impact on regular winds. For example, as warm air rises near the Equator, it flows toward the poles. In the Northern Hemisphere, these warm air currents move to the right as they travel north. In other words, they bend east.

Impact on Human Activity

The Coriolis effect shapes airplane routes. As we have seen, wind directions are largely set by the Coriolis effect. For this reason, airplane pilots have to understand how the effect works when planning flight paths. The same is true for rockets.

The Coriolis Effect Closer to Home

Here's one last example of the Coriolis effect at work. You can actually try it without superhuman strength. Suppose you and a friend are throwing a ball to each other while on a merry-go-round. When the merry-go-round is still, throwing the ball is easy. When the merry-go-round is rotating, the ball won't reach your friend unless you throw it extra hard. Normally, the ball will curve to the right.

In reality, the ball is flying in a straight line. It's you and your friend who are moving out of its way while the merry-go-round is spinning.

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

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