## SciTech

### What makes a hurricane spin and what ‘The Simpsons’ got wrong

Dr. Anantha Aiyyer is an associate professor of marine, earth, and atmospheric sciences at the N.C. State University. Here he explains why hurricanes and tropical cyclones spin the way they do. Hint: It has nothing to do with the direction your toilet bowl flushes. Questions and answers have been edited.

Q: Why do hurricanes go counterclockwise in the northern hemisphere and clockwise in the southern hemisphere?

A: You can think of a hurricane as a spinning column of air that stretches from sea-level up to about 45,000 feet. Over much of this depth, air flows counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. To understand why, let’s consider the rotation of the earth. We know that the earth spins along an invisible axis from west to east. But if we look at it differently, something interesting jumps out. Imagine you are hovering above one of the poles and are looking down at the earth. Pick the North Pole and do this thought experiment. In what sense is the earth rotating? Counterclockwise. Now fly to the South Pole and look at the earth and notice its rotation. It will help to use a spinning tennis or basketball to visualize this. It now appears to spin clockwise.

We note that hurricanes spin in the same direction as the earth’s hemisphere (as viewed from the poles) they are located in. This is not a coincidence. The atmosphere is attached to the spinning earth and as a result every column of air is also spinning. When that inherent spin is concentrated (this happens when air convergences into an area) we get rotating storms like hurricanes. As the sense of rotation is opposite in the two hemispheres, this leads to oppositely spinning hurricanes.

This phenomenon is related to the “Coriolis effect” that occurs due to the rotation of the earth. This effect causes objects to be deflected in a direction perpendicular to their motion. You can see that if a moving object (like a spinning top) keeps turning at 90 degrees, it will soon trace a circular path. The Coriolis effect deflects objects in opposite directions in the two hemisphere because – you guessed it – it depends on the perceived spin of the earth in the respective hemispheres.

Q: Does it have anything to do with the direction your toilet bowl flushes?

A: If memory serves right, this was a recurring theme in “The Simpsons” episode “Bart vs. Australia” when Lisa invoked the Coriolis effect to argue that water swirls down a sink or toilet differently Down Under. Sadly she was wrong! The Coriolis effect is felt when objects move long distances on a rotating planet. A toilet flush is nearly instantaneous compared to the slow rotation of the earth and the distance traveled by water is very small. While there may be a minute Coriolis effect, it is not enough to impact the direction of water flow. To summarize: air flowing in a hurricane or a jet stream is significantly impacted by Coriolis effect; but water being flushed or baseballs hit in a stadium – not quite. We’ll just chalk this to an urban myth.