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The Coriolis Effect

Hurricanes and the Coriolis Effect

Figure 1.

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Shorter distance to travel in the same amount of time means slower speeds closer to the poles.

Credit: NOAA/JPL-Caltech

Figure 3.

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Representation of the air flow around a low-pressure area (in this case, Hurricane Isabel) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows. Take note of the counter-clockwise rotation.

Credit: Titoxd, CC BY-SA 3.0

   The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around the Earth. The Coriolis effect is responsible for many large-scale weather patterns. ​

    The Coriolis Effect is named after French mathematician and physicist Gaspard-Gustave de Coriolis. The Coriolis Effect affects weather patterns, it affects ocean currents, and it even affects air travel. As important as the Coriolis Effect is, many have not heard about it, and even fewer understand it. In simple terms, the Coriolis Effect makes things (like planes or currents of air) traveling long distances around the Earth appear to move as a curve as opposed to a straight line. It’s a pretty weird phenomenon, but the cause is simple: Different parts of the Earth move at different speeds.

   Think about this: It takes the Earth 24 hours to rotate one time (Figure 1). If you are standing a foot to the right of the North or South Pole, that means it would take 24 hours to move in a circle that is about six feet in circumference. That’s about 0.00005 miles per hour. Hop on down to the equator, though, and things are different. It still takes the Earth the same 24 hours to make a rotation, but this time we are traveling the entire circumference of the planet, which is about 25,000 miles long. That means you are traveling almost 1040 miles per hour just by standing there.

   So even though we are all on Earth, how far we are from the equator determines our forward speed. The farther we are from the equator, the slower we move.

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   Good question! Now think about this: You are on a train traveling at top speed and you are passing a train that is moving a bit slower (Figure 2. You see, for some mysterious reason, that there is a soccer goal on this slower train. Always prepared, you happen to have a soccer ball handy and want to make an impressive trick shot. You take an incredible shot directly at the goal when you are even with the slower train. Even though your aim is dead-on, the ball travels to the side and misses the net. That’s because the ball is traveling not only in the direction of the goal, but it is also going in the direction (and speed) of your train.

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    One of the most important things the Coriolis Effect acts on are storm systems. Big storms like hurricanes and typhoons (tropical cyclones) are low-pressure systems. That means that they suck air into their center.

    But as we just learned, air traveling long distances across Earth does not simply move in a straight line. Just like our soccer ball, the air being sucked into the storm deflects (Figure 3). This deflection is what causes tropical cyclones to spin.

    Another thing the Coriolis Effect does is make these massive storms rotate in different directions in the Northern and Southern Hemispheres.

Take a look at our bird’s-eye view picture of trains to the left (Figure 4). You will notice that the ball kicked on the north side of the tracks appears to travel to the right from the viewpoint of the kicker. The ball kicked to the south, though, appears to the kicker to travel to the left. That’s no mistake. That’s actually what happens, and it applies to any large distance movement throughout each hemisphere. The result? Storms in the Northern Hemisphere spin counterclockwise and those in the Southern Hemisphere spin clockwise.

Okay. So How Does That Prevent Things from Traveling in a Straight Line?

Hurricanes and the Coriolis Effect

Figure 2.

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This is what happens with our attempted trick shot.

Credit: NOAA/JPL-Caltech

Figure 4.

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View from above.
Credit: NOAA/JPL-Caltech
Credit: Coriolis Effect (NOAA  SciJinks)
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