When an object is flying through the air, its course might suddenly change. When that happens, we say the object has been deflected. The Coriolis effect is a natural event in which objects seem to get deflected while traveling around and above Earth.

The planet Earth is constantly rotating, or spinning, from west to east. Every 24 hours, it completes a full rotation. This rotation causes the Coriolis effect. The key to the effect is that different points on Earth rotate at different speeds. Points on the Equator rotate faster than points near the poles. The Equator is an imaginary line around the middle of Earth. It divides the globe into Northern and Southern Hemispheres.

Earth is wider at the Equator. In order to make a full rotation in 24 hours, points near the Equator have to cover a larger distance than points near the poles. So points near the Equator have to move faster. They race at nearly 1,600 kilometers (1,000 miles) an hour, while points near the poles move much slower.

How does Earth's rotation cause the Coriolis effect? Let's pretend you're standing at the Equator. Imagine throwing a ball to a friend in the middle of North America. If you throw the ball in a straight line, it will seem to land to the right of your friend. Remember, he's moving east more slowly than you are. When you throw the ball, the ball is moving toward your friend, but it's also moving east at a faster speed than he is.

Now let's pretend you're standing at the Pole">North Pole. When you throw the ball to your friend, it will again seem to land to his right. This time, it's because he's moving east faster than the ball is. No matter where you are in the Northern Hemisphere, the ball will deflect to the right.

The Coriolis effect has a large impact on the weather. When air currents travel across large areas, they act like the ball in our imaginary game of catch. They seem to bend to the right in the Northern Hemisphere. In the Southern Hemisphere, currents bend to the left. In general, the Coriolis effect is stronger with higher speeds or longer distances.

Weather Patterns

Cyclones are an example of the influence of the Coriolis effect. A cyclone is a large air mass that rotates around a center. As they rotate, cyclones suck air into their center, or "eye." The air currents are pulled in from all directions. In the Northern Hemisphere, they are then deflected to the right. As a result, the cyclone seems to rotate counterclockwise. In the Southern Hemisphere, currents are deflected to the left. As a result, cyclones seem to rotate clockwise.

The Coriolis effect also helps shape regular wind patterns. For example, warm air near the Equator flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right, or east, as they move northward.

Impact on Human Activity

Wind directions are largely determined by the Coriolis effect, so airplane pilots have to understand this effect when planning flight paths. The same is true for rockets.

Bullets are affected, too. Sometimes military snipers have to be aware of the Coriolis effect. Snipers often target a single person surrounded by others. Even a small deflection could hurt innocent people.

The Coriolis Effect Closer to Home

There are simple ways to see the Coriolis effect in action. For example, say you and a friend are throwing a ball back and forth while sitting 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 make it to your friend unless you throw it extra hard. Thrown normally, the ball will seem to curve, or deflect, to the right. In reality, the ball is moving in a straight line. It's you and your friend on the merry-go-round who are moving out of its path.