When an object is moved off its course, we say it has been deflected. The Coriolis effect describes how objects that are not connected to the ground seem to get deflected as they travel long distances around Earth.

The Coriolis effect is responsible for many large-scale weather patterns. The key to the Coriolis effect lies in the planet's rotation from west to east. Specifically, Earth rotates faster at the Equator than it does at the poles. Every point on Earth takes the same amount of time—24 hours—to make a complete rotation. Yet points along the equator have to travel a much longer distance than points near the poles and, therefore, move at a higher speed. Equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour, whereas near the poles the planet rotates at a slow 0.00008 kilometers (0.00005 miles) per hour.

These different speeds of rotation are the reason for the Coriolis effect. Let's pretend you're standing at the Equator and want to throw a ball to your friend in the middle of North America. As you throw the ball, both you and the ball are already moving eastward more quickly than your friend is. So if you try to throw the ball in a straight line, it will land to the right of your friend. Remember, your friend in North America is moving east at a slower speed.

Now let's pretend you're standing at the North Pole. When you throw the ball to your friend, it will again appear to land to the right of him. This time, it's because he's moving faster than you are and has moved ahead of the ball. Everywhere you play this game of global catch, in the Northern Hemisphere the ball will deflect to the right.

This apparent deflection is called the Coriolis effect. Fluids traveling across large areas, such as air currents, are affected in the same way as the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents bend to the left.

The strength of the Coriolis effect depends on velocity, or speed of travel in a particular direction. It depends both on the velocity of Earth and the velocity of the object or fluid being deflected. The higher the speed or the longer the distance, the stronger the Coriolis effect.

Weather Patterns

Certain weather patterns, such as cyclones and trade winds, are due to the Coriolis effect. A key factor is air pressure, or the averaged weight of air molecules pressing down on Earth. Weather systems can be low-pressure or high-pressure, and air tends to move from high-pressure to low-pressure areas.

Cyclones are low-pressure systems that suck air into their center, or eye. In the Northern Hemisphere, as fluids move from high-pressure systems to low-pressure systems, they are subject to the Coriolis effect. In other words, they aim for low-pressure systems, but pass them to the right. As air masses are pulled into cyclones from all directions, the storm system—a hurricane—seems to rotate counterclockwise. In the Southern Hemisphere, the opposite happens: currents are deflected to the left, and storm systems seem to rotate clockwise.

Outside of storm systems, the Coriolis effect also helps define regular wind patterns around the globe. As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right, or east, as they move northward. Then the currents descend back toward the ground and slowly move from the northeast to the southwest, back toward the Equator. The constantly circulating patterns of these air masses are known as trade winds.

Impact on Human Activity

Airplanes and rockets are impacted by the Coriolis effect as well. The directions of prevailing winds are largely determined by this effect, so pilots must take it into account when charting flight paths over long distances.

The Coriolis effect can also change bullet trajectories. Sometimes, military snipers have to consider the effect since even a small deflection could injure innocent people.

The Coriolis Effect on Other Planets

Earth rotates fairly slowly compared to other planets. The slow rotation of Earth means the Coriolis effect is not strong enough to be seen at slow speeds over short distances. Jupiter, on the other hand, has the fastest rotation in the solar system. On Jupiter, the Coriolis effect actually transforms north-south winds into east-west winds, some traveling more than 610 kilometers (380 miles) per hour. The divisions between winds that blow mostly to the east and those that blow mostly to the west create clear horizontal divisions, called belts, among the planet's clouds. The boundaries between these fast-moving belts are highly active storm regions. The 180-year-old Great Red Spot is perhaps the most famous of these storms.

The Coriolis Effect Closer to Home

There are some realistic ways of observing the Coriolis effect. 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 not rotating, throwing the ball is easy. While the merry-go-round is rotating, however, the ball won't make it to your friend without significant force. Thrown with regular effort, the ball appears to curve to the right. Another friend who is standing off of the merry-go-round will be able to tell you what's really happening. The ball is moving straight through the air, but you and your friend are rotating and moving out of its path.