- #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.
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.