Perhaps you've heard that hurricanes, ocean gyres and flushing toilets all swirl in one direction in the Northern Hemisphere — counter-clockwise — while in the Southern Hemisphere, they swirl in the the opposite direction. Well, it's mostly true, though there's some myth attached to these claims, especially regarding the flushing toilets — we'll get to that in a bit!
The reason behind these phenomena is a rather dizzying force known as the Coriolis effect. Named after the 19th-century French mathematician Gustave Coriolis who first explained it, the Coriolis effect is not your typical force. In fact, it's sometimes referred to as a "pseudo-force," though not in a derogatory fashion. Unlike how we usually think of forces, the Coriolis effect does not arise from any physical interaction between two objects. Rather, it emerges from a rotating reference frame. In this case, that reference frame is the rotation of Earth on its axis.
Basically, as the Earth twists on its axis, everything attached to the Earth gets dragged along with it. Since we're attached to the Earth ourselves most of the time (except when we're jumping, or flying in a plane or some other flying apparatus), we don't intuitively perceive that we're spinning around with the Earth, even though we are. Objects that aren't tethered to the Earth, however, will travel in a straight motion over the turning planet's surface.
Like playing ball on a spinning carousel
This is part of what can make the Coriolis effect so dizzying and anti-intuitive. To get a better grasp of this effect, imagine that you're tossing a ball to a friend while the two of you are riding on opposite sides of a spinning playground carousel. As soon as you release the ball, it's no longer being dragged by the movement of the carousel, so it will travel in a straight line from the point where you released it. But since you and your friend are still spinning along, the ball will appear as though some force is pulling it on a curve to the right or left depending on the direction in which you're spinning.
The carousel is like the Earth, and the wind is like the ball. This is how the Coriolis effect can impact weather systems. In the Northern Hemisphere, wind from high-pressure systems pass low-pressure systems on the right, causing the system to swirl counter-clockwise. Meanwhile, the opposite is true in the Southern Hemisphere, which is why large weather systems swirl clockwise there.
Because ocean currents are driven in part by the movement of wind over the water's surface, the Coriolis force also affects the movement of ocean currents. Though the circulation is not as significant as that in the air, it's this deflection that plays a large role in the formation of the world's large ocean gyres. The spiraling effect of these gyres, meanwhile, influences the formation of hurricanes — but it all starts with the Coriolis effect.
Interestingly, pilots plotting flight paths from one point on Earth to another must also take the Coriolis effect into consideration. Since a plane ceases to be tethered to the spinning Earth when it takes off, a pilot must consider the plane's movement relative to the Earth turning. A plane can't simply fly in a straight line toward its destination, otherwise it will curve away much in the same way that the ball appears to curve when thrown from one side of the spinning carousel to the other.
What about the toilets?
The Coriolis effect is also the commonly cited cause of toilets flushing in opposite directions depending on where you live. The only problem? Toilets don't actually flush in opposite directions in the opposing hemispheres. Such claims to the contrary are myths.
The reason the Coriolis effect isn't observed in toilets is simply that the effect is very weak. The Earth completes only one rotation a day, which is rather plodding compared to many other planets. So while the Coriolis effect can be observed among really large systems, and in dramatic fashion, it's often overpowered by other forces in smaller systems. For instance, most toilets flush by forcibly injecting water at an angle, which is far more powerful an effect than Coriolis can supply.
This doesn't mean that the Coriolis effect has no influence whatsoever on your toilet water, however. If all other forces were controlled for, as some experimenters have demonstrated, the water in even small systems will slowly but eventually twist in the direction predicted by the Coriolis effect when drained. This method is not recommended for powering your toilet, but it does show that there is at least some truth, however minuscule, behind the myth.
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