What is the direction a low pressure system moves in the Northern Hemisphere?

Although the Coriolis effect doesn't determine the direction of toilet flushes or knuckle balls, it does have a significant impact on weather patterns. If you've ever watched The Weather Channel, you've probably noticed all those arrows swirling around the meteorologist's map to indicate wind direction. The direction those arrows are pointing toward is largely determined by the Coriolis effect.

If the Earth didn't rotate, winds would travel either north or south due to differences in temperature and pressure at different latitudes. But since the Earth does rotate, the Coriolis force deflects these winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

In the Northern Hemisphere, this deflection causes the wind flow around high pressure systems to go clockwise while flow around low-pressure systems travels counterclockwise. Imagine a low-pressure system as a vacuum that sucks all the surrounding air straight towards it, creating many vectors of wind that all focus on one spot. Because of the Coriolis effect, each of these vectors gets twisted to the right, which in turn creates a counterclockwise flow. With a high-pressure system, air gets forced outward and the Earth's eastward spin creates a clockwise flow. In the Southern Hemisphere, the opposite takes place: Wind around low-pressure systems circles clockwise while wind around high-pressure systems circles counterclockwise.

The swirling motions around low-pressure systems are actually the driving forces behind hurricanes. The air gets sucked in with such force and spins to such a degree that a potentially destructive storm develops. Warm ocean water fuels the system and if it gets a chance to grow over a period of time, powerful winds of more than 62 miles per hour (100 kilometers per hour) can form a storm strong enough to destroy anything in its path -- all just from the spin of our little planet.

The curvature of the winds created by the Coriolis effect also helps create surface ocean currents. The wind drags on the water's surface, creating spiral currents called gyres. As you may have guessed, the gyres in the Northern Hemisphere spin clockwise and the ones in the Southern Hemisphere spin counterclockwise.

Meteorologists and sailors aren't the only ones who have to contend with the Coriolis effect. Since aircraft cover large distances in a short period of time, pilots must also take its influence into account when charting the paths for their flights. For instance, a plane headed from Miami (where the Earth's rotation is more pronounced) to New York would end up in the Atlantic Ocean if the pilot ignored the effects of the Earth's rotation.

The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground 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 Earth’s rotation. Specifically, Earth rotates faster at the Equator than it does at the poles. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour.

Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up.

Now let’s pretend you’re standing at the North Pole. When you throw the ball to your friend, it will again to appear to land to the right of him. But this time, it’s because he’s moving faster than you are and has moved ahead of the ball.

Everywhere you play global-scale "catch" in the Northern Hemisphere, the ball will deflect to the right.

This apparent deflection is the Coriolis effect. Fluids traveling across large areas, such as air currents, are like the path of 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 appear to bend to the left.

The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances.

Weather Patterns

The development of weather patterns, such as cyclones and trade winds, are examples of the impact of the Coriolis effect.

Cyclones are low-pressure systems that suck air into their center, or “eye.” In the Northern Hemisphere, fluids from high-pressure systems pass low-pressure systems to their right. As air masses are pulled into cyclones from all directions, they are deflected, and the storm system—a hurricane—seems to rotate counter-clockwise.

In the Southern Hemisphere, currents are deflected to the left. As a result, storm systems seem to rotate clockwise.

Outside storm systems, the impact of the Coriolis effect 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 (east) as they move northward. The currents descend back toward the ground at about 30° north latitude. As the current descends, it gradually moves from the northeast to the southwest, back toward the Equator. The consistently circulating patterns of these air masses are known as trade winds.

Impact on Human Activity

The weather impacting fast-moving objects, such as airplanes and rockets, is influenced by the Coriolis effect. The directions of prevailing winds are largely determined by the Coriolis effect, and pilots must take that into account when charting flight paths over long distances.

Military snipers sometimes have to consider the Coriolis effect. Although the trajectory of bullets is too short to be greatly impacted by Earth’s rotation, sniper targeting is so precise that a deflection of several centimeters could injure innocent people or damage civilian infrastructure.

The Coriolis Effect on Other Planets

The Earth rotates fairly slowly, compared to other known planets. The slow rotation of Earth means the Coriolis effect is not strong enough to be seen at slow speeds over short distances, such as the draining of water in a bathtub.

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 incredibly active storm regions. The 180-year-old Great Red Spot is perhaps the most famous of these storms.

The Coriolis Effect Closer to Home

Despite the popular urban legend, you cannot observe the Coriolis effect by watching a toilet flush or a swimming pool drain. The movement of fluids in these basins is dependent on manufacturer’s design (toilet) or outside forces such as a strong breeze or movement of swimmers (pool).

You can observe the Coriolis effect without access to satellite imagery of hurricanes, however. You could observe the Coriolis effect if you and some friends sat on a rotating merry-go-round and threw or rolled a ball back and forth.

When the merry-go-round is not rotating, rolling the ball back-and-forth is simple and straightforward. While the merry-go-round is rotating, however, the ball won’t make to your friend sitting across from you without significant force. Rolled with regular effort, the ball appears to curve, or deflect, to the right.

Actually, the ball is traveling in a straight line. Another friend, standing on the ground near the merry-go-round, will be able to tell you this. You and your friends on the merry-go-round are moving out of the path of the ball while it is in the air.