By Don Lincoln, Ph.D., University of Notre Dame
The Coriolis effect is the reason objects flying or flowing above the Earth’s surface deflect from their originally intended direction. The effect is strongest at the poles and weakest at the equator. If an object starts flying northward or southward from the equator, it will be deflected eastward due to the Earth’s counterclockwise rotation.
(Image: 3d_and_photo/Shutterstock)
The Coriolis effect is realized in how Earth’s rotation affects the direction of objects flying long distances. An example can help us understand the Coriolis effect best. If there existed a bow and arrow big enough to shoot across the globe, one could stand around the equator and shoot it northward, aiming to hit the north pole. If the arrow was shot from Texas and traveled in a line of constant longitude, you’d imagine that it would fly over Oklahoma, Nebraska, South Dakota, North Dakota, and on into Canada. However, in reality, this does not happen.
In reality, the arrow would probably fly over New York State and eventually land somewhere in the Atlantic Ocean. The arrow is deflected rightward to the east due to the earth’s Coriolis effect — the force that does not obey falling and throwing rules in physics.
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The Coriolis Effect
In 1651, the Italian scientist Giovanni Battista Riccioli realized that the Earth’s rotation would make a cannonball shot northward deflect towards the east. However, the final name of the effect came from the French physicist Gaspard-Gustave de Coriolis. He published a paper in 1835 about the forces on the rotating parts of industrial machines, particularly water wheels. It took until 1920 for the modern name to become common, but it did. Now it is called the Coriolis effect.
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Earth’s Rotation Speed
By using some simple physics equations, the rotation speed of the Earth can be calculated. The circumference of the Earth at the equator is about 25,000 miles. Besides, the Earth rotates once every 24 hours to make a full rotation. Consequently, the Earth at the equator is moving at around 1042 miles per hour.
An object four feet away from the North Pole travels only a circle of around 24 feet in circumference. The speed in the middle of the northern hemisphere, i.e., around Minneapolis and Minnesota, is a little more than 700 miles per hour. The same speeds apply for the southern hemisphere. Due to the rotation direction, everything moving from the equator to the poles moves eastward: rightward in the northern hemisphere and leftward in the southern one. The air is also something flowing around the Earth.
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Why Hurricanes Rotate
Hurricanes are a result of the Coriolis effect. Thus, they rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Regardless of the air pressure in different areas, air flows into the center and is deflected rightward. Within the hurricane, the rotating air also wants to deflect to the right. Hence, the hurricane finds a size limit due to air resistance to new incoming air.
There is a constant tension between the pressure differential, making the air flow inward. The outward pressure is also due to the Coriolis effect. The air pressure difference regulates the velocity of the winds. Further, the strength of the Coriolis force is proportional to wind speed, and this interplay sets a limit on the size of the hurricane. As the Coriolis effect is very weak at the equator, no hurricanes happen there. What else realizes the effect?
The Foucault Pendulum
Even though some people believe the water in the toilet rotates proportionate to the Coriolis effect, it is too small a movement to show it. The water does rotate different directions in the northern and southern hemispheres, but that is due to the different directions of the jets. What can show the effect is the Foucault pendulum, invented in 1851 by the French physicist Léon Foucault.
It is made of a very heavy weight hanging from a long cable, moving back and forth like other pendulums. The difference is that due to its size, the change in its oscillation direction can also be seen.
Conclusively, the Earth’s rotation is the reason hurricanes happen, and it can deviate other flying objects from their path as well.
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Common Questions about the Coriolis Effect
Earth’s rotation is the main reason for the Coriolis effect. The effect deflects anything that flies or flows over a long distance above the ground, proportionate to Earth’s spin direction. Even storms can be a result of the rotation; hence, they do not form similarly everywhere on Earth.
The Coriolis effect refers to how a moving object shifts direction rightward in the Northern hemisphere and leftward in the Southern hemisphere. This is the reason hurricane winds turn left in the Northern hemisphere. The effect can be seen in large-scale movements.
The Coriolis effect is the result of Earth’s counterclockwise spin around its axis. Thus, it is strongest at the Earth’s poles. The further one moves away from the poles; the lower becomes the Coriolis effect. The effect pushes flying or flowing objects eastward in both hemispheres, but the direction is different in each, depending on the object’s destination location.
Even though the water in toilets spins different directions in the northern and southern hemispheres, it is not a result of the Coriolis effect. To see the effect, one needs a much bigger scale, such as hurricanes. The reason water rotates different directions in northern and southern hemispheres when flushing, is the different direction of the jets. The Coriolis effect does exist, but it is too small to see on such a small scale.
