Tornadoes and angular momentum

In summary, the conversation discusses different explanations for the formation and maintenance of tornadoes. One person finds the explanation of angular momentum perplexing and suggests that other factors, such as differential air speeds and pressure, may play a role. They also mention that the Met Office provides a description of the mechanism behind tornado formation, but others may need a deeper understanding of fluid dynamics to fully explain the process. The conversation ends with a clarification that angular momentum alone cannot explain circular motion without a centripetal force.
  • #1
Zebulin
8
3
I explained this thinking to a meteorologist once and she couldn't give me an answer. Any physicists want to give it a shot?

I find the typical explanation of tornadoes perplexing (that's a polite way of saying I don't believe it). The explanations I've seen claim that tornadoes start out horizontal but then become vertical, and they keep spinning during this change because of angular momentum (or they have other behaviors base on angular momentum). How does that make sense?

When you start a stop spinning and it keeps going on its own, you can say that the reason it keeps spinning is because it has angular momentum. Angular momentum is essentially a combination of linear momentum and the solidness of the top which means that there is a centripetal force on all the parts.

However, in a fluid, that explanation does not work because there is no centripetal force between the molecules. A vortex often forms in a fluid when the container has a hole in it allowing the fluid to escape, but if anyone asks why the fluid continues to swirl as long as the hole is there, surely the answer would not be "angular momentum". The answer is probably something like, "the vortex is the most stable motion of the fluid that allows the fastest possible flow out of the container". If you plug the hole, then the swirling must stop the moment there is no longer the low-pressure center to act like a centripetal force. The molecules shoot off in straight paths.

There is a likely explanation already available for the low-pressure core of a tornado: tornadoes form in conditions with cold, dry air moving over warm, humid air. The cold, dry air is heavier so this is unstable. A vortex that was acting as a conduit transporting the lighter air upward above the heavier air could have the necessary low-pressure center. No need for angular momentum. But even if that's not where the low pressure comes from, you still need to explain the centripetal force that holds the tornado together, and once you do, there is no point to talking about angular momentum.
 
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  • #2
Zebulin said:
Angular momentum is essentially a combination of linear momentum and the solidness of the top which means that there is a centripetal force on all the parts.
Angular momentum doesn't require a centripetal force or that all parts move on circles. A system of two masses moving along two parallel lines in opposite directions also has angular momentum.
 
  • #3
Zebulin said:
The explanations I've seen claim that tornadoes start out horizontal but then become vertical, and they keep spinning during this change because of angular momentum (or they have other behaviors base on angular momentum). How does that make sense?
https://www.metoffice.gov.uk/learning/wind/tornado/how-are-tornadoes-formeddescribes the mechanism. What is there not to believe about that model? I have respect for the Met Office and I don't see any good reason to disbelieve what they say. The initial horizontal axis for rotation comes directly from convection and differential air speeds. Then the disturbance to tilt the vortices is reasonably explained. They use no Maths in that description but then, neither do you. You would need to know some fluid dynamics to explain how vortices are formed but there are many hits on Google on the topic. Pick one to match your state of knowledge and level of Maths.
Btw, you don't need attraction to keep particles moving in a circle. A greater pressure on the outside of a vortex and reduced pressure on the inside will do that job. The pressure difference will depend on the speeds of the air.
 
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  • #4
A.T. said:
Angular momentum doesn't require a centripetal force or that all parts move on circles. A system of two masses moving along two parallel lines in opposite directions also has angular momentum.

I don't see how that is relevant. The angular momentum of such a system is not going to make the masses start moving in a circle around each other. The whole point is that you can't use angular momentum as an explanation for why circular motion continues if there is no centripetal force.
 
  • #5
sophiecentaur said:
I have respect for the Met Office and I don't see any good reason to disbelieve what they say.
Apparently you misunderstood the point of my question. I was not asking what authorities I should trust; I was asking for an explanation.

sophiecentaur said:
The initial horizontal axis for rotation comes directly from convection and differential air speeds. Then the disturbance to tilt the vortices is reasonably explained. They use no Maths in that description but then, neither do you. You would need to know some fluid dynamics to explain how vortices are formed but there are many hits on Google on the topic. Pick one to match your state of knowledge and level of Maths.
Btw, you don't need attraction to keep particles moving in a circle. A greater pressure on the outside of a vortex and reduced pressure on the inside will do that job. The pressure difference will depend on the speeds of the air.

From my question, it should be clear that I already know all of this. I don't see anything here that is relevant to answering my question.
 
  • #6
Thread closed temporarily for Moderation...
 
  • #7
Thread is re-opened for now.

Zebulin said:
Apparently you misunderstood the point of my question. I was not asking what authorities I should trust; I was asking for an explanation.
This reply is not really appropriate. He is not asking you to trust an authority. I believe he is just pointing out that the source is probably allowed under the PF rules for reputable sources.
Zebulin said:
From my question, it should be clear that I already know all of this. I don't see anything here that is relevant to answering my question.
Well, folks are just trying to help you. Do you have a background in fluid mechanics that we can use in proposing sources? If not, it may be harder to find sources that give you a satisfying explanation...
 
  • #8
Zebulin said:
I don't see how that is relevant. The angular momentum of such a system is not going to make the masses start moving in a circle around each other. The whole point is that you can't use angular momentum as an explanation for why circular motion continues if there is no centripetal force.
Let the two particles or masses moving past each be surrounded by a whole bunch of other particles. There is no where else for the masses to go except ( they can't keep continuing forward since that position is occupied ) sideways to the void left by the other mass. And voila, linear notion with its associated angular momentum becomes circular motion. In a continious fashion this becomes the vortex.
 
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  • #9
Good responses so far in the thread. Also paging @boneh3ad
 
  • #10
Zebulin said:
I don't see how that is relevant. The angular momentum of such a system is not going to make the masses start moving in a circle around each other.
The difference in wind velocities at different altitudes starts a spinning motion that is not likely to remain completely horizontal. Especially with updrafts and downdrafts, there is bound to be a great deal of spinning around a vertical axis.
The whole point is that you can't use angular momentum as an explanation for why circular motion continues if there is no centripetal force.
I don't see how centripetal force is relevant. I think that your reliance on centripetal force to explain angular momentum is fundamentally wrong.
EDIT: I think I realize what you mean. The centripetal force is needed to prevent the molecules from flying off in straight lines from center of rotation rather than going in a circle. But the low pressure at the center of a tornado would essentially provide what is needed to prevent that.
 
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  • #11
Zebulin said:
From my question, it should be clear that I already know all of this.
It wasn't very clear at all. You quoted an idea from an unspecified source (conversation in a pub?) and then a lack of knowledge about vortex formation.
We can only go on what we read in members' posts.
 
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  • #12
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  • #13
A.T. said:
As already noted, the centripetal force in a spinning fluid is provided by the radial pressure gradient.

As for why the fluid starts spinning:
https://en.wikipedia.org/wiki/Kelvin–Helmholtz_instability


I wonder why the guy in the video didn't refer to a Mackerel Sky. You can see a hint of that very often in good weather.
upload_2018-4-13_0-1-27.jpeg

From this site
 

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  • #14
A.T. said:
As already noted, the centripetal force in a spinning fluid is provided by the radial pressure gradient.

As for why the fluid starts spinning:
https://en.wikipedia.org/wiki/Kelvin–Helmholtz_instability


Are you suggesting that the characteristic rotating motion in a severe storm's updraft is caused by K-H instability?
 
  • #15
olivermsun said:
Are you suggesting that the characteristic rotating motion in a severe storm's updraft is caused by K-H instability?
It's a general concept how velocity shear initiates rotation. The tornado specifics are explained in the link in post #3.
 
  • #16
olivermsun said:
Are you suggesting that the characteristic rotating motion in a severe storm's updraft is caused by K-H instability?
The formation of a single hurricane vortex at the centre of a depression / cyclone can be explained in terms of Coriolis effect but I think the small scale effect in tornados is below the Coriolis scale. Once it has been formed, in many ways, a vortex is a vortex.
 

1. What is angular momentum and how is it related to tornadoes?

Angular momentum is a measure of the rotational motion of an object around a fixed point. In the context of tornadoes, angular momentum refers to the spinning motion of the air within a tornado. This spinning motion is what gives tornadoes their characteristic funnel shape.

2. How does angular momentum affect the strength of a tornado?

The greater the angular momentum of a tornado, the stronger it typically is. This is because higher levels of angular momentum indicate a more intense spinning motion, which can lead to faster wind speeds and more destructive force in a tornado.

3. Can angular momentum be used to predict the behavior of tornadoes?

Yes, scientists use mathematical models to analyze the angular momentum of tornadoes and predict their behavior. However, predicting the exact path and strength of a tornado is still a challenging task.

4. How does the Earth's rotation affect the angular momentum of tornadoes?

The Earth's rotation plays a significant role in the formation and movement of tornadoes. The Coriolis effect, which is caused by the rotation of the Earth, influences the direction and spin of the air within a tornado, ultimately affecting its angular momentum.

5. Are there any real-world applications of studying tornadoes and angular momentum?

Studying tornadoes and angular momentum can help us better understand the dynamics of severe weather and improve tornado forecasting and warning systems. It can also inform building codes and disaster preparedness measures in tornado-prone areas.

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