Have you ever flown in an airplane? If you have, then what did you do during your flight? Did you read a book? Watch a movie? Sleep? Whatever you did, you probably trusted that the pilot knew what they were doing, and that they would get you to the correct destination.

Did you consider though that while you were flying through the air, your destination was also moving? This is because the Earth was rotating. Your pilot not only had to know where your destination was, they also had to know where your destination would end up. Where it was moving to. So that they could arrive at the correct location. In other words, your pilot had to account for the rotation of the Earth so that the plane and the destination would both end up at the same place at the same time.

The result is that instead of flying in a straight line, they must fly the plane in a curved line matching the rotation of the Earth. We call this the Coriolis Effect.

The Coriolis Effect is responsible for many things on Earth. It is the reason why hurricanes spin in the way that they do. The air above the ocean near the equator is moving faster than the air near the poles. This difference in speed creates a force called pressure gradient force. The Coriolis Effect deflects this force to the right in the northern hemisphere and to the left in the southern hemisphere. This is why hurricanes spin counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

The Coriolis Effect also affects how fluids move in general. It is sometimes used to explain why water spins down a drain in different directions in different hemispheres, though something as small as a drain is not really big enough to be impacted by this effect. It can however be used to explain the path of large ocean currents.

Why does the Coriolis Effect cause this deflection? Let’s say we are looking at the northern hemisphere. The Earth is rotating counterclockwise when viewed from above the North Pole. So, anything moving north will appear to move to the right when viewed from above. This is because it is being deflected by the rotation of the Earth. Similarly, in the southern hemisphere, anything moving north will appear to move to the left when viewed from above, again because of the deflection caused by the rotation of the Earth.

Imagine if a child stood on the North poll and threw a ball towards the south. The ball would appear to curve to the right when viewed from above. If the child threw the same ball with the same force from the South Pole, it would appear to curve to the left when viewed from above.

Because the atmosphere is not attached to the Earth, and is instead floating above it, it acts like these balls. As the air moves, the ground beneath it rotates away. Which makes the air appear to deflect or curve. This is why Hurricanes in the northern hemisphere spin counterclockwise, and why in the southern hemisphere they spin clockwise.

The Coriolis Effect is a result of the rotation of the Earth. It affects moving objects, fluids, and anything else that is in motion. It is responsible for many things on our planet, from the direction of hurricanes to the path of ocean currents. The next time you are flying in an airplane, or watching a hurricane spin, remember the Coriolis Effect.