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Author Topic: Why is a hurricane round?  (Read 9824 times)

Offline Chemistry4me

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Why is a hurricane round?
« on: 14/12/2008 02:54:36 »
After watching "the day after tomorrow". I'm wondering why hurricanes are round? Why is there an eye in the middle. Also why is it the coldest in the middle/eye? At least that was my impression after the movie.


 

paul.fr

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Why is a hurricane round?
« Reply #1 on: 14/12/2008 05:23:00 »
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After watching "the day after tomorrow". I'm wondering why hurricanes are round?

The simple answer is because they are spinning.


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Why is there an eye in the middle.

Not quite as simple, but the physics guys would probibly tell you it's to do with the conservation of angular momentum.


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Also why is it the coldest in the middle/eye? At least that was my impression after the movie.

I've not seen the film, but from memory (not to be trusted) I thought it was generally warmer at the eye! I can't remember if this is something to do with latent heat from the eye wall, or simply because things are so much calmer within the eye with clear skies. Or I could simply be wrong...
 

paul.fr

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Why is a hurricane round?
« Reply #2 on: 14/12/2008 06:08:36 »
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I can't remember if this is something to do with latent heat from the eye wall,

erm, I think you can forget I said that bit.
 

Offline Chemistry4me

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Why is a hurricane round?
« Reply #3 on: 14/12/2008 07:48:39 »
Thanks Paul. But surely, its not just because they are spinning?
 

paul.fr

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Why is a hurricane round?
« Reply #4 on: 14/12/2008 16:12:40 »
Thanks Paul. But surely, its not just because they are spinning?

No, but that was the easy answer! :-) . If nobody else replies i will, when my internet is up and running (don't ask!), totally unable to do long posts on the pda.
 

lyner

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Why is a hurricane round?
« Reply #5 on: 14/12/2008 21:50:32 »
A cylinder is probably the most stable (lowest energy state) basic shape for a vortex. The angular momentum would have to keep changing if it were not circular motion? The air would have to keep speeding up and slowing down and the pressure would also keep changing. That could only happen if there were some form of resonance. Would it be possible to have a resonance at such a low frequency as the rotational frequency of the column?
 

Offline LeeE

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Why is a hurricane round?
« Reply #6 on: 15/12/2008 00:18:23 »
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Also why is it the coldest in the middle/eye? At least that was my impression after the movie

This highlights the dangers of learning science from movies  :D

I like this film a lot but the bit you're referring to there is just artistic license.  In the film, it's explained that the eye of the storm draws down super-cooled air from the stratosphere, and this accounts for the sudden intense freezing.  In real life however, while we experience much colder conditions in the upper atmosphere, when compared with ground level, the temperature of the individual molecules of air at high altitudes is actually higher than at ground level and we only feel colder because there is so little air.

So if the eye of the storm really were to suck down air from the upper atmosphere, not only would the air molecules be hotter, but you'd also have air moving from a low-pressure region to a high-pressure region, which doesn't work.
 

Offline Chemistry4me

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Why is a hurricane round?
« Reply #7 on: 15/12/2008 00:34:59 »
So why is there an eye in the middle? And whats this angular momentum that you talk of?
 

lyner

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Why is a hurricane round?
« Reply #8 on: 15/12/2008 09:41:41 »
Just North of the centre, the air will be traveling West and, just South of the centre, the air will be traveling East. There must be a point where the air is going nowhere - i.e be stationary.
Because of the friction, there will be a 'velocity gradient' - where the speed changes gradually from zero in the middle and gets faster as you go out. (This doesn't go on for ever,  of course).

Angular momentum:
For an object rotating around a centre, its angular momentum is given by its mass times its rotation speed (angular velocity) times the radius of rotation squared. It's a quantity which stays the same in an isolated system (it is 'conserved).

When a spinning skater brings her arms inwards, the radius of rotation of her arm has reduced so the angular speed will increase to maintain the same angular velocity. If you could haul the Moon closer to the Earth, its orbit time would reduce for the same reason.

Likewise, if the air rushing round in a hurricane were to change its radius of rotation, its speed would have to change. The most stable situation is for it to keep to the same radius.
 

Offline Chemistry4me

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Why is a hurricane round?
« Reply #9 on: 22/12/2008 22:55:31 »
Just getting back to the movie, why did all those hurricanes form in the first place? I remember that man saying something about fresh and salt water content, although I've got no idea how that contributed. Can anyone who has seen that movie tell me whats going on?
 

Offline LeeE

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Why is a hurricane round?
« Reply #10 on: 23/12/2008 17:37:34 »
There are circulating currents in the world's oceans that flow between the hot tropical regions and the colder temperate and polar latitudes.  The water in the tropics is heated and then flows north (and south but there is relatively little land south of the tropics to separate the currents), where the water is colder, and as it does so the heat it has absorbed in the tropics is released, raising the temperature of the air in the temperate and polar latitudes and thus reducing the difference in temperature between the tropics and the temperate/polar regions.

The premise of the film is that freshwater from melting glaciers reduces the salinity of the seawater in the temperate/polar regions and this upsets the circulating ocean currents that are carrying heat from the tropics because the depth at which the currents flow, which affects how much heat it absorbs/releases depends upon both the temperature and salinity of the water.  The upset of these circulating ocean currents leads to an increased temperature difference between the tropics and the temperate/polar regions, with the tropics getting hotter and the temperate/polar regions getting colder.

Where you've got two adjacent masses of air at different temperatures they will try to mix, to even out the difference between them, and to mix the air must move, which means winds, and one of the factors that effects how fast they mix is the amount of difference between them.  If there's only a small difference between the two regions the air movement will be relatively slow and sedate but if the differences are great then the air movement will be faster and more energetic.

So the result of freshwater from melting glaciers upsetting the ocean currents leads to a greater temperature differential, which in turn leads to stronger winds.

Now because the Earth is a spheroid, the cold air masses at northern (and southern) temperate/polar regions are traveling at a  different speed to the warm air in the tropic regions and at the boundary between them, where they mix, the air is rotated between the two regions.  You can get an idea of how this happens by holding a pencil between the palms of your hands and then moving both hands slowly away from you, at slightly different speeds - the pencil will roll between your palms.

The final result then, is higher than normal winds in the form of large spinning storms along the boundary between the warm and cold air masses.

At least, that's the scenario the film appears to be presenting.
 

Offline Chemistry4me

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Why is a hurricane round?
« Reply #11 on: 23/12/2008 22:59:33 »
That is very interesting LeeE, thank you for explaining! Is this what is actually happening as a result of Global Warming or just something for the cinemas?   
 

Offline LeeE

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Why is a hurricane round?
« Reply #12 on: 24/12/2008 12:50:34 »
The key aspect of the film scenario, that of polar meltwater affecting heat carrying ocean currents, is plausible but there's not, as yet, proof that it is occurring or will occur.  Everything else in the film is based upon that single factor.

In real life, the situation is more complex.  There are many factors that affect how these systems work, some of which we believe we understand well, some of which are poorly understood and then there are probably some of which we are still unaware or of whose effects and influence we can only guess.  In the end, it's impossible to say anything with certainty, except that if things are changing there will be change.
 

paul.fr

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Why is a hurricane round?
« Reply #13 on: 17/01/2009 23:37:11 »
Thanks Paul. But surely, its not just because they are spinning?

short and (very) simple...

As warm, moist air over an ocean rises upward and is replaced by cooler air, an area of low pressure is created. Surrounding air of higher pressure rushes in and that too becomes warm and moist and rises. As the warm air continues to rise, more surrounding air swirls in to take its place. As this air rises and cools it form the clouds you see in the depression.This system of clouds and wind spins and grows, fed by the oceans heat and water evaporating from the surface. Not to mention the Coriolis Force.

Click through this animtion for more help in understanding why hurricanes spin

If you have any friends who are UK residents, then the Met Office send out free dvd's explaining this and more.
 

paul.fr

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Why is a hurricane round?
« Reply #14 on: 17/01/2009 23:44:14 »
oooh. If you want an easy demonstration of coriolis than get yourself down to the local park / playground with a ball (foot, basket or tennis ball). Sit on the roundabout and have someone spin you very fast, then push the ball alond the floor of the roundabout and watch the direction of the ball.
 

Offline Chemistry4me

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Why is a hurricane round?
« Reply #15 on: 18/01/2009 08:10:33 »
Unfortunately it is dark outside right now so can you just tell me what will happen?
 

Offline Chemistry4me

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Why is a hurricane round?
« Reply #16 on: 18/01/2009 08:18:09 »
Thank you for the link why hurricanes spin it was very helpful.

 

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Why is a hurricane round?
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