Sep. 02, 2011

Science Dad on Hurricanes

by Vince Harriman

Click to enlarge images

As soon as we finished cleaning up the mess we left on the front porch from our Earthquake experiments, we had to take everything off the porch to prepare for the hurricane! I hope everyone is recovering well from Hurricane Irene and is prepared for the possibility of Hurricane Katia or others making landfall.

One thing we did last week and last weekend was watch hours and hours of news reports and satellite footage of Hurricane Irene churning counterclockwise up the coast. It seemed as good a time as any to talk with Beckett about huge weather events such as hurricanes and what shapes and directs them.

Almost all hurricanes and tornadoes in the Northern Hemisphere spin counterclockwise due to something called the Coriolis effect. The Coriolis effect was first described by the French mathematician Gaspard-Gustave Coriolis, who observed the deflection of moving objects when they are viewed in a rotating reference frame such as the Earth.

Most people know the urban legend that the Coriolis effect causes toilets to drain counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere, so we decided to test this theory and asked friends around the world to help us out. We even had a real scientist (Science Friday blogger Kayla Iacovino) record a video down at McMurdo Station in Antarctica!

As you can see, the results were rather mixed. We learned that even if a science experiment is large and takes place in both hemispheres on several different continents, you don't always get a conclusive result (or one that lets you fully accept or reject your hypothesis). Turns out, it is a common misconception that the Coriolis effect is responsible for the direction that water drains from sinks or tubs. While the Coriolis force affects large bodies like air masses (or smaller bodies over long periods of time), it does not have a noticeable effect on water masses on the the scale of your toilet.

But the global toilet experiment did get Beckett and I thinking about the direction that cyclones spin.

In this time-lapse video of Hurricane Irene, created by NOAA, notice the perfect counterclockwise rotation from start to finish.
NOAA Environmental Visualization Laboratory - GOES Satellite Tracks Hurricane Irene.

Now take a look at this animation of Brazil's first recorded cyclone, recorded in March 2004. This cyclone spins clockwise!
NOAA Environmental Visualization Laboratory - Imagery Archive - Brazil's First Cyclone.

Cyclones in the northern hemisphere spin counterclockwise, while cyclones in the southern hemisphere spin clockwise.

There are a lot of complicated factors that contribute to the spin of hurricanes, cyclones, and tornadoes, but the easiest way to understand the Coriolis part of it is by imagining the rotating forces in two dimensions -- as on a wheel, a merry-go-round, or carousel. If you are standing in the center of the merry-go-round or carousel, you don't feel the motion that much, no matter how much the thing spins. But walk out to the edge, and you feel every bit of the speed. The area near the edge of the spinning platform is traveling farther each rotation than the area closer to the axis. This is similar to what happens with giant meteorological formations -- in the Northern hemisphere, the speed of the earth near the equator, at the southern end of the pressure differential, is much higher than it is at the northern end. So, in the Northern hemisphere, when air moves from an area of higher pressure towards a low pressure center, it does not go straight; the Coriolis force deviates the air to the right. You can see a schematic representation of these forces above.

In addition to the Coriolis force, there are four other things necessary to produce a storm or hurricane that spins:

  • the ocean water must be warm enough to provide energy;
  • the atmosphere in the area must be unstable and changeable;
  • there must be high humidity;
  • and wind shear must be present low enough in the atmosphere to impact the storm.

If the storm forms too far from, or too near, the equator, there will not be enough Coriolis forces to start the storm spinning into a cyclonic event.

So, check the weather -- Katia is slowly growing and may be on the East Coast sometime next week. We'll be watching the satellite imagery to see the spin.


About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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