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Out in Nature

Coriolis Effect in Hurricanes


The Coriolis effect is a feature of rotating systems wherein objects seem to be pushed to one side as they move. Though commonly called a force, it is really just an effect of rotating frames. It is important in phenomena at long length scales in rotating systems, and is known to govern the dynamics of ocean currents, the formation of hurricanes, and even the rotation of sunspots. In this set, we'll explain the origin of the effect and showcase its effect or lack-there-of in several applications.

Imagine standing on the edge of a merry-go-round as a friend throws you an apple from the center. If the merry-go-round is moving slowly, then your friend can throw the apple straight to you without much trouble. However, if the merry-go-round is spinning very quickly, your friend will need to take your rotation into account. If they throw the ball straight at your current position, you'll be gone by the time it gets there. This is because—at the edge—you're traveling quickly to the side, while your friend stands in place.

If you were to throw the apple from the edge of the merry-go-round toward the center, the same would happen, except that the effect would be in the opposite direction. This is the essential Coriolis effect.

What it amounts to is an acceleration toward the side. Quantitatively, its magnitude can be shown to be proportional to \(\omega v\) where \(\omega\) is the speed at which the merry-go-round rotates, and \(v\) is the speed at which the apple is thrown.

From our simple example, we can see that this deflection is insignificant when the merry-go-round rotates slowly compared to the motion of the apple. Conversely, when the merry-go-round rotates very quickly relative to the motion of the apple, the effect is quite significant.

A turntable, as shown in the diagram, is rotating in the clockwise direction with a constant angular velocity \(\omega\). A boy is standing on the turntable at point A (facing the point O) holding a ball in his hand. If he throws the ball directly towards the center, then he will see the ball

Two kids are sitting across from each other, on opposing sides of a merry-go-round. If both of them toss a ball toward the other, which of the following is true?

As we mentioned in the context of the merry-go-round, the Coriolis effect is significant when rotation is significant compared to the movement of an object. For this reason, humans don't usually experience the Coriolis effect except in the contrived context of merry-go-rounds or perhaps when throwing things from moving vehicles as they travel around a tight curve.

This is because the difference in rotation between points on Earth's surface is negligible except at long distances (i.e. you and your neighbors experience almost the same rotation due to Earth's spin, whereas the difference between someone in Toronto and someone in Panama City is enormous). Therefore, human experience takes place on a length scale that's too small for the Coriolis effect to manifest (\(\ell_\textrm{human} \ll \ell_\textrm{Coriolis}\)).

By contrast, events like ocean currents, or atmospheric flows take place on very long length scales, and are therefore strongly affected by the Coriolis force. As we mentioned before, it has a strong influence on the spin of hurricanes.

In which direction do hurricanes rotate in the Northern Hemisphere?

A popular claim is that the direction of the spiral in draining bathtubs and toilets is due to the Coriolis effect. Can this be true?

What is the influence of the Coriolis effect on hurricanes that form at the Equator?

Here, we saw how rotating systems give rise to a fictitious deflective acceleration, and learned how to gauge when it is a significant factor in any given situation. On the merry-go-round it is hard to ignore, but thankfully it has no effect on basketballs, since the length of the court is so big compared to the difference in the rotation of the ends of the court. At long length scales its effects cannot be ignored, and indeed determine the rotation of large scale phenomena like hurricanes and global weather patterns.

Though we've laid out the essentials of the Coriolis effect, reference frame effects are at the heart of physics and indeed the nature of reality. General relativity redefined physics in the twentieth century by showing the long scale effects of energy on spacetime, and it would seem that the current century is due for another such awakening regarding the quantum nature of space itself.


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