Coriolis force is a force experienced by objects moving on the surface of the earth in a direction that is not along the direction of Earth's rotation. Since the earth rotates from west to east, any object that is not moving in this direction, will experience the Coriolis force. This force has been observed to affect large cyclonic storms and causes them to rotate clockwise in the southern hemisphere and counterclockwise in the northern hemisphere.
Though this is a real, observable scientific effect, there is a widespread myth associated with it. In this post I am going to tackle this myth head on.
What is the myth?
The myth is that due to the Coriolis force, water in your sink drains clockwise in the southern hemisphere while it drains counterclockwise in the northern hemisphere. There is a video that demonstrates this strange phenomenon.
If you have read the description of the Coriolis force there is a good chance that the myth would sound true to you. Your opinion would have been further strengthened by the video. But if you understand the physics behind the Coriolis force, I am confident that you would be able to conclude that the demonstration is a trick.
What is the Coriolis force?
The earth rotates around its axis once every 24 hours (approximately). Since it is uniformly rotating along the eastward direction (west to east) different parts on Earth are moving eastward at different velocities (VEast). This is because at the equator, the earth has to cover a much longer distance to complete one full rotation, while at a higher latitude, say 30 deg N, the distance that needs to be covered is much shorter. Since the total time taken for one full rotation is a constant while the distance covered is different, the earth spins the fastest at the equator (about 1600 KPH). It gets slower and slower as we move towards the poles and at the poles, VEast is zero.
Let us now consider that there is a low depression at 30 deg N latitude. In order to equalize the pressure, air from regions of higher pressure would rush in. As we already know, the air near the equator has a greater VEast compared to the air at the poles. This means that air at the equator carries an extra bit of VEast compared to the air at 30 deg N latitude. Therefore, when air flows from the equator to 30 deg N latitude it moves to the right (eastward) (This is similar to a passenger getting off a moving bus. He carries an extra velocity along the direction of the moving bus compared to the ground. So he runs for sometime before he comes to rest). But the low pressure bends the air back to the left and hence it follows a trajectory as shown below. Air from the north pole has a much lower VEast compared to the air at 30 deg N latitude. Therefore it tries to catch up and ends up moving westward. Once again the low pressure bends the air back to the right. This results in an overall effect of the air moving counterclockwise in the northern hemisphere. In the southern hemisphere, the direction is reversed.
Does the Coriolis force affect anything on Earth?
As described above, Coriolis force has a large effect on the direction of winds, like hurricanes, because hurricanes can be several hundred kilometers in diameter. What about objects that don't move large distances? Let's take the example of a tennis ball moving across the court during a tennis match.
Let us imagine a tennis court with the server standing on the equator and the receiver is up north, standing 78 feet (28.78 m) away (That's the length of a tennis court). If the Coriolis force affects the tennis ball when served, then the ball must always curve to the right before it reaches the receiver. Is this true? In short, the answer is "No". There are other forces that determine the direction of the ball, like the direction of the force imparted by the racquet on the ball, the direction of spin and others. Why is this so? VEast, where the server stands is, only one part in a hundred million more from the VEast where the receiver stands. Hence the Coriolis force is negligible over the length of a tennis court. If the Coriolis force was the dominant force in the tennis court, tennis would be a predictable, boring game. The same is true with most ball sports.
The Coriolis force only affects objects that are moving over hundreds of miles. So do you think that the water in your sink would be affected by it? The tennis court is about 30 m in length, yet there is no Coriolis force on the tennis ball. Hence one can easily guess that the water in a 30 cm wide sink will not be affected by the Coriolis force. So the answer to the question "Which direction does the water in your sink drain" depends mostly on external forces that arise due to the tap not being aligned with the drain or due to the basin being uneven, which can give rise to eddy currents (the same backward currents that can be seen when a water stream hits a rock) and not on where you live on Earth.
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| Tropical storm Leslie off the coast of North America captured by NASA on August 30th 2012. Courtesy: Jeff Schmaltz, LANCE MODIS Rapid Response Team, Goddard Space Flight Center |
Though this is a real, observable scientific effect, there is a widespread myth associated with it. In this post I am going to tackle this myth head on.
What is the myth?
The myth is that due to the Coriolis force, water in your sink drains clockwise in the southern hemisphere while it drains counterclockwise in the northern hemisphere. There is a video that demonstrates this strange phenomenon.
If you have read the description of the Coriolis force there is a good chance that the myth would sound true to you. Your opinion would have been further strengthened by the video. But if you understand the physics behind the Coriolis force, I am confident that you would be able to conclude that the demonstration is a trick.
What is the Coriolis force?
The earth rotates around its axis once every 24 hours (approximately). Since it is uniformly rotating along the eastward direction (west to east) different parts on Earth are moving eastward at different velocities (VEast). This is because at the equator, the earth has to cover a much longer distance to complete one full rotation, while at a higher latitude, say 30 deg N, the distance that needs to be covered is much shorter. Since the total time taken for one full rotation is a constant while the distance covered is different, the earth spins the fastest at the equator (about 1600 KPH). It gets slower and slower as we move towards the poles and at the poles, VEast is zero.
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| Animated gif by Wikiscient available under a Creative Commons Attribution-Noncommercial license |
Does the Coriolis force affect anything on Earth?
As described above, Coriolis force has a large effect on the direction of winds, like hurricanes, because hurricanes can be several hundred kilometers in diameter. What about objects that don't move large distances? Let's take the example of a tennis ball moving across the court during a tennis match.
Let us imagine a tennis court with the server standing on the equator and the receiver is up north, standing 78 feet (28.78 m) away (That's the length of a tennis court). If the Coriolis force affects the tennis ball when served, then the ball must always curve to the right before it reaches the receiver. Is this true? In short, the answer is "No". There are other forces that determine the direction of the ball, like the direction of the force imparted by the racquet on the ball, the direction of spin and others. Why is this so? VEast, where the server stands is, only one part in a hundred million more from the VEast where the receiver stands. Hence the Coriolis force is negligible over the length of a tennis court. If the Coriolis force was the dominant force in the tennis court, tennis would be a predictable, boring game. The same is true with most ball sports.
The Coriolis force only affects objects that are moving over hundreds of miles. So do you think that the water in your sink would be affected by it? The tennis court is about 30 m in length, yet there is no Coriolis force on the tennis ball. Hence one can easily guess that the water in a 30 cm wide sink will not be affected by the Coriolis force. So the answer to the question "Which direction does the water in your sink drain" depends mostly on external forces that arise due to the tap not being aligned with the drain or due to the basin being uneven, which can give rise to eddy currents (the same backward currents that can be seen when a water stream hits a rock) and not on where you live on Earth.
How did the trickster do it?
It should be clear to you by now that the demonstration was a trick. To know how it was done, go back to the video and look for external forces. It might be be difficult to figure it out due to the camera angle. But once you have watched the video again, make sure that you read this article by Phil Plait (@BadAstronomer) for the correct answer.
Have fun figuring out the trick!
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