What Causes Fast Winds in Hurricanes?

[Bjarne]

Hurricanes – Why so fast Winds?

Which force is responsible for the very fast wind in a hurricane, and why is the fastest velocity close to the eye? – why not in the periphery ?

 

[turin]

Which force is responsible for the very fast wind in a hurricane, ...

There are many forces. I would say, in a grand sweeping generalization, that the Coriolis force is responsible. However, I hate to call that a force, and prefer to call it the Coriolis effect. Another mechanism that must come into play is the pressure differential generated by the deviation from the Maxwell-Boltzmann distribution in the atmospheric density, which is in turn caused by an anomalous temperature gradient.

... why is the fastest velocity close to the eye? – why not in the periphery ?

Short answer: conservation of mass and momentum. The air flows into the eye, which has a smaller circumference that the outer edge. So, it gets squeezed in and travels faster. It's basically just a funneling (nozzling) effect. The same basic principle applies to putting your thumb on the end of the water hose to make the water squirt out faster.

 

[Bjarne]

There are many forces. I would say, in a grand sweeping generalization, that the Coriolis force is responsible.

How?
I mean today the Coriolis force are there too.
But the clouds on the sky / wind are calm.

 

[DaveC426913]

Wiki can provide a http://en.wikipedia.org/wiki/Hurricane#Mechanics".

Among other things, it points out that the Coriolis Effect is not the main source of energy; the main source of energy is the heat released from condensation of the rising moist air mass. The Coriolis Effect merely makes it spin.

 

[Bjarne]

the main source of energy is the heat released from condensation of the rising moist air mass

Sounds more logical yes...

BUT think about wind speed 300 Km/h or more
Hmmm... energy from rising moist "only”?
It sounds to me like still some force is missing here.
Why this relative huge acceleration ??
I mean inner and outer diameter doesn’t make that much difference?

 

[DaveC426913]

Sounds more logical yes...

BUT think about wind speed 300 Km/h or more
Hmmm... energy from rising moist "only”?
It sounds to me like still some force is missing here.

Did you read the article?

Scientists at the US National Center for Atmospheric Research estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (10^18 J) per day, equivalent to about 1 PW (10^15 watt). This rate of energy release is equivalent to 70 times the world energy consumption of humans and 200 times the worldwide electrical generating capacity, or to exploding a 10-megaton nuclear bomb every 20 minutes.

I'm thinkin' 300km/h winds is pretty tame considering the energy levels involved. It sucks the heat out of the ocean over thousands of square miles.

Why this relative huge acceleration ??
I mean inner and outer diameter doesn’t make that much difference?

Of course it does. Watch water drain down the sink. The rotation at the centre is marked enough to actually create a funnel (water held against gravity by centrifugal force) compared to the outer edge, where the water might be almost still.

 

[Bjarne]

No doubt that the rising heat is parts of the force involved.
And no doubt it's a huge amount of energy involved...

BUT what make me wonder is relative slow raising heat ends up with huge wind velocity.
What exactly is responsible for such huge wind acceleration / velocity..?
I mean think about from the moment air begins to rotate slowly around a low pressure and then speed up day by day. Why this day by day increasing acceleration?

 

[DaveC426913]

No doubt that the rising heat is parts of the force involved.
And no doubt it's a huge amount of energy involved...

BUT what make me wonder is relative slow raising heat ends up with huge wind velocity.
What exactly is responsible for such huge wind acceleration / velocity..?
I mean think about from the moment air begins to rotate slowly around a low pressure and then speed up day by day. Why this day by day increasing acceleration?

When ten cubic kilometres of air decides it wants to be five kilometres higher up in the atmosphere, then there's a ten cubic kilometre hole to fill in a hurry.

And it is a positive feedback cycle. The more air rises, the more wind; the more wind, the more evaporation; the more evaporation, the more energy.

 

[Bjarne]

When ten cubic kilometres of air decides it wants to be five kilometres higher up in the atmosphere, then there's a ten cubic kilometre hole to fill in a hurry.

And it is a positive feedback cycle. The more air rises, the more wind; the more wind, the more evaporation; the more evaporation, the more energy.

Yes it make sence.
Thank's

 

[granpa]

what makes you think that the rising warm air is slow? the speed depends on the pressure difference and a 10 km high column of warm air can create quite a pressure difference.

 

[Bjarne]

what makes you think that the rising warm air is slow? the speed depends on the pressure difference and a 10 km high column of warm air can create quite a pressure difference.

The confusion is properly that we do not know why a low pressure starts to rotate.
A hurricane (that reach USA) begins with very weak rotating thunder clouds near Africa.
It takes days before strong wind begins to occur.

Basically if the rotation would not occur, don’t you think it is difficult to imaging heat rising with 300 km/h?

I mean after the rotation has started it is easy to understand that we will have a self-perpetuating process.

 

[DaveC426913]

The confusion is properly that we do not know why a low pressure starts to rotate.
A hurricane (that reach USA) begins with very weak rotating thunder clouds near Africa.
It takes days before strong wind begins to occur.

Basically if the rotation would not occur, don’t you think it is difficult to imaging heat rising with 300 km/h?

I mean after the rotation has started it is easy to understand that we will have a self-perpetuating process.

We do know what causes the rotation - the Coriolis Effect. This effect occurs because the system is hundreds of miles across. And when you take a giant mass of air that's rotating at a slow, stately pace, and then compress it into a space that's one tenth or one hundredth the size (i.e. near the eye), the angular momentum is conserved, which means the slow stately winds will multiply ten-fold or a hundred-fold.

 

[LOKI777]

it rotates because the Earth rotates, winds are higher in the center because of the vacuum in the center, stronger and faster winds cause a smaller eye because of the squeeze.

 

[turin]

Nice to see that you've come back around, Dave.

 

[DaveC426913]

Nice to see that you've come back around, Dave.

Come back around to what?

 

[Bjarne]

We do know what causes the rotation - the Coriolis Effect.

This is both correct and false.

We do know the cause of the rotation when we speak about a certain size.

BUT when a small low pressure (and thunder clouds) begins to rotate (near by Africa) and travels over the Atlantic, the low pressure area are initially too small to that we can't blame the Coriolis force to be involved.

And the answer here is we do not know why small low pressures begins to rotate.

At least this is what was claimed at a scientific film I recently saw..

 

[DaveC426913]

This is both correct and false.

We do know the cause of the rotation when we speak about a certain size.

BUT when a small low pressure (and thunder clouds) begins to rotate (near by Africa) and travels over the Atlantic, the low pressure area are initially too small to that we can't blame the Coriolis force to be involved.

And the answer here is we do not know why small low pressures begins to rotate.

At least this is what was claimed at a scientific film I recently saw..

It is still due to conservation of angular momentum. A draining sink does the same thing without benefit of Coriolis Effect.

 

[turin]

Come back around to what?

It was a pun - apparently a bad one.

 

[turin]

BUT when a small low pressure (and thunder clouds) begins to rotate (near by Africa) and travels over the Atlantic, the low pressure area are initially too small to that we can't blame the Coriolis force to be involved.

And the answer here is we do not know why small low pressures begins to rotate.

I would suggest the gigantic Atlantic anticyclone, generated from the Antilles high, that spurs off the huricanes that are ripe for the plucking from the Saharan rainstorms. The edge of the anticyclone pushes the rainstorms into an opposite rotation, like a sun gear causes planetary gears to spin in opposite directions.