WHAT'S UP: Tornadoes are spun up by Earth’s rotation

Apr 29, 2011

Tornadoes twist because Earth rotates. Our overall weather patterns would be much different if Earth didn’t spin once about every 24 hours.

Large-scale wind currents — the winds we feel every day — are first generated by pressure differences in the atmosphere. Like all fluids, air flows naturally over level terrain from areas of high pressure to areas of low pressure. That’s why air quickly rushes out of a car tire when it is punctured: the pressure inside the tire is significantly higher than outside.

Our atmosphere is not spatially uniform. Large areas of slightly higher than average pressure and areas of slightly lower than average pressure are distributed here and there across the oceans and continents, and air flows develop between them. These air flows tend to smooth out the large-scale pressure variations that cause them.

But high- and low-pressure systems persist for days and weeks, so some contributing effect must act to sustain the pressure differences while the wind is trying to equalize them.

One of the dominate effects that helps high- and low-pressure systems persist is the so-called Coriolis effect, named after the French scientist who first described it in 1835.

All objects, large and small, including tiny air molecules, move in a straight line at constant speed if left to themselves. Left to themselves, air molecules would rush straight from a high-pressure region to a low-pressure region to quickly equalize the pressure. But that’s not what happens here on Earth.

On Earth, in the Northern hemisphere, moving bodies apparently get deflected to the right — always to the right, here in the Northern hemisphere.

In the Northern hemisphere, as air flows into a low-pressure center to “fill it up,” the persistent deflection to the right builds up a wind field that spirals counter clockwise around the low. And low-pressure centers, then, don’t get “filled up” as quickly as they would without this persistent rightward deflection.

The rightward deflection of moving objects in the Northern hemisphere (leftward in the Southern hemisphere), Coriolis noted, is an apparent oddity that crops up because the Earth spins. Isolated moving objects really do move in straight lines, but fixed to the ground of a spinning Earth, our reference frames spin out from underneath, causing us to perceive deflections from straight-line paths.

One experiment I have always wanted to try is to watch drunks stumble out of a bar late at night. If there is anything the Coriolis effect, more Northern-hemisphere drunks would stumble to the right than to the left.

One thing you may have heard about the Coriolis effect that isn’t true is that it causes sink and bathtub drains — and toilets — to spin counter clockwise in theNorthern hemisphere and clockwise in the Southern hemisphere. Testing this notion would be a great science-fair project.

It turns out that residual random motions of stumbling drunks and water in sinks, bathtubs and toilets can cause them to spin and fall either way.

The Coriolis effect is not noticeable on the small scales of our everyday lives; however, on flows of air in the atmosphere it can build up to tremendous proportions.
In the Northern hemisphere, all hurricanes and nearly all tornadoes spin counter clockwise because of the Coriolis effect.  

— By Doug Furton, a member of the physics faculty at GVSU.  Send questions and suggestions to dgf@inbox.com.  An archive of some of his “What’s up” columns is available online at http://gegenschein.wordpress.com.

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