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What Is the Coriolis Effect?

Figure showing forward speed of rotation at different points on Earth.

A Powerful “Force”

Drawing of Gustave Coriolis.

The Coriolis Effect is named after French mathematician and physicist Gaspard-Gustave de Coriolis.

It 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 at 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.

What Do You Mean Parts of Earth Move at Different Speeds!?

Think about this: It takes the Earth 24 hours to rotate one time. 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.

Figure showing forward speed of rotation at different points on Earth.

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

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.

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

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. 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.

Animation of scenario described in text.

This is what happens with our attempted trick shot.

Let’s pretend these trains are the Earth at different latitudes and add another red train. Think of the two red trains as the northern and southern tropics and the blue train as the equator. The red trains are going slower than the blue train. Remember, the farther you go from the equator, the slower you move.

Now let’s put our trains on an actual Earth-shaped globe:

Animation of scenario described in text.

The trains still move at different speeds, but now they would appear to travel parallel to each other.

Even though the red trains are going slower than the blue train, since they are traveling a shorter distance, they would appear from a bird’s-eye view to be going at the same speed. That doesn’t mean your trick shot would behave any differently though.

From a bird’s-eye view, it would look like this:

Animation of scenario described in text.

A bird's eye view.

And that’s the deflection we are talking about! Anything traveling long distances, like air currents, ocean currents pushed by air, and airplanes, will all be deflected because of the Coriolis Effect! Weird, right?