Unmasking the Coriolis Effect:

Hurricane Madeline, Pacific Ocean: Photo courtesy of the National Weather Service

An Invisible Influence on Wind and Water

You have probably heard of the Coriolis Effect. It’s a rather cool concept in Earth science that might seem like magic, but it’s all about physics. This invisible force explains why hurricanes spin and why long-distance pilots have to be careful about their flight paths. I used to teach about it in my Earth science unit. But I just learned that I was wrong about an important fact.

The Coriolis Effect describes how objects that aren’t connected to the ground, like air currents or thrown balls, seem to curve as they move across large distances on the Earth. It’s all because of our planet’s rotation. The Earth spins faster at the Equator than at the poles, so if you try to throw a ball in a straight line over a long distance, it’ll appear to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis Effect in action.

You can see this happen on a playground merry-go-round that is in motion. If you try to toss a ball to a friend directly opposite, the ball may seem to veer off to the side. The ball leaves your hand and continues to move in a straight line, just like Newton’s first law of motion says. Since the merry-go-round is moving, however, your friend has shifted position relative to the earth.

You’ve probably seen the Coriolis Effect on the weather report. When the weather reporters talk about hurricanes or cyclones, they’re really talking about massive demonstrations of the Coriolis Effect. In the Northern Hemisphere, these storms rotate counterclockwise, and in the Southern Hemisphere, they rotate clockwise, all due to the way the Earth’s rotation affects air currents.

And it’s not just the weather. The Coriolis Effect even impacts how pilots chart long-distance flights, as the Earth’s rotation can cause subtle shifts in their course. In extreme cases, even people shooting a rifle at targets a mile away have to take the Coriolis Effect into account. For nearby targets or short flights, the Coriolis effect has no influence. The scale has to be big enough for the rotation of the earth to be a significant factor.

This is where I made my mistake. I had fallen for the common myth that water draining from a sink will swirl counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis Effect does not determine the way water drains from your sink or toilet. That’s usually influenced by the design of the sink or toilet and tiny movements in the water. The Coriolis effect is too weak at the scale of a household sink to make any difference. I remember passing on this myth as if it were fact in some conversations. I certainly hope I did not teach it to any of my students. And if I did, I hope this see this correction and apology.

So, next time you see a hurricane on the news or watch a long-haul flight take off, remember the Coriolis Effect, the invisible force that subtly shapes our world. And, also, remember to take what a teacher or textbook says with a grain of salt!

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