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Wind Weight

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Skiffman View Drop Down
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Post Options Post Options   Quote Skiffman Quote  Post ReplyReply Direct Link To This Post Topic: Wind Weight
    Posted: 17 Nov 05 at 1:24pm

In the 4th November issue of YnY, Andy Rice was talking about wind weights and americas cup. A reader has sent an email in this month explaining that it is not possible, I wondered what other people thought.

Myself I think that there is such a thing as wind weight. Because last year the 29er Worlds were in Silvaplana which is 1850m above sea level were the air is less dense and i remember the first day reading 25knots average my anemometer when only felt about 18knots. Get back to the UK doing some winter training at weymouth (-13 degrees windchill) with anemometer reading 18knots when it felt like 25knots. Or am i just stupid and the wind is always the same?

Also the reason why the anemometer will read the same wind speed for 2 different air densitys is because the air/paritcles in the air is going that speed but the particles are more spread out in the less dense air so there are less particles hitting your sail even though there hitting it at the same speed therefore it feels like less wind?????

what do other people think?

I dont really know but find it quite interesting

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Pierre View Drop Down
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Post Options Post Options   Quote Pierre Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 05 at 3:14pm

Good question and I just found this by googling...

Wind does not have weight, but the air that wind moves
does have mass and thus weight.  The wind does exert force
and that force is what moves the air molecules.  What you
feel as wind is the force of the air molecules moving
against you.


David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

Looks about right to me.  The operative word is MASS.  Thinner air has less mass.



Edited by Pierre
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Stefan Lloyd View Drop Down
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 05 at 3:56pm

Originally posted by Skiffman

Also the reason why the anemometer will read the same wind speed for 2 different air densitys is because the air/paritcles in the air is going that speed but the particles are more spread out in the less dense air so there are less particles hitting your sail even though there hitting it at the same speed therefore it feels like less wind?????

At 2000m height the air density is 20% less than at sea level. So you would definitely feel less force in the sails at the same wind speed compared to sea level.

The opposite applies to a lesser extent with cold air. If are hardy enough to sail at 3C, the air is 10% denser than on a hot summer day of 30C. So the same wind speed will feel "heavier" in winter. Lots of people claim to observe this but I'm not really certain a 10% difference in force would be that noticeable. The force exerted by the breeze is proportional to the square of the windspeed, so a 5% change in windspeed causes a 10% change in force (more or less). 5% change in windspeed is the difference between 20 and 21 knots, and how many of us would really be able to tell the difference between a steady 20 and 21 knots?

 

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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 05 at 10:10pm
Ahhhh,

But there is also more to it than this.

We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s) (well on most boats anyway) on our boats that creat low pressure that "suck" us along....

Heahache time in terms of actually "calculating" if colder = faster and hotter/higher/thinner = slower.....  you never know, you might find that thinner (but faster air) might reward the more skillfull.
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CurlyBen View Drop Down
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Post Options Post Options   Quote CurlyBen Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 05 at 11:24pm
Wouldn't denser air have a greater pressure differential anyway though?
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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 05 at 11:38pm
Originally posted by CurlyBen

Wouldn't denser air have a greater pressure differential anyway though?


Yes.....

but there was some tongue in cheek

Heahache time in terms of actually "calculating" if colder = faster and hotter/higher/thinner = slower.....  you never know, you might find that thinner (but faster air) might reward the more skillfull.
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CurlyBen View Drop Down
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Post Options Post Options   Quote CurlyBen Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 05 at 11:46pm
Oops.. I'm a bit slow today! However for an identical difference in pressure (I assume this is what you meant to calculate?) then wouldn't the lower density air have marginally less drag force on the rig/hull/crew? Can't see that itself for the variations mentioned would be a huge difference! If that wasn't the calculation you meant I'll go and sit in the corner..
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 05 at 7:42am

Originally posted by Scooby_simon

We don't use the "force" of the air to move us along; we use the pressure difference as a result of the speed differences across the front and the back of the sail(s)

And the pressure difference results in a force. Force = pressure x area. No force, no movement (Newton's 2nd law).

 

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Post Options Post Options   Quote Entyplod Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 05 at 6:59pm
Stefan Lloyd is pretty much right on this issue:

Wind on a cold day will cause the sails of a boat to provide more force than on a warm one (assuming the atmospheric pressures both days are equal). The confusion lies in both that the difference is not that large, and also in what a cup anemometer is actually measuring.

The force exerted by wind on sails is determined by the kinetic energy of the wind; being proportional to the density of the air, times the square of the wind speed. For any given wind speed denser air has greater kinetic energy, and hence gives a greater force on the sails, and vice versa.

For a boat the differences are however small. Roughly; for a 3 degree change in temperature (or a 10mb change in pressure) the density of air changes by 1%. Take a 20kt wind on a day that is 20degrees C. On a 5 degrees C day (15 degrees colder) the same air would have a density 5% higher. Hence the cold air would exert the same force as the air on the warm day would if its speed is the square route of 1.05 (i.e. 1.025) times higher – :this gives 20.5kts, not a big difference!!

A cup anemometer is trying to measure the speed of the wind, not the force (more correctly the ‘dynamic pressure’) produced by it. Ideally the cups spin at the speed the air flows past them – in reality this does not quite happen due to friction in the anemometer. If you want to measure the force (dybamic pressure) generated by air you need something that works more like an aircraft's air speed indicator.

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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 05 at 7:56pm

An anemometer is measuring the speed of the wind not the force.  I'd rather be hit by a fly at 30mph than a car at 30mph.  The difference is caused by the mass.  It seems perfectly reasonable to me that if we bend wind of greater mass we get greater force even though the wind speed is the same.

Now somebody said we'd be unlikely to be able to notice a 10% difference in force - I'd have to disagree.  When competing against other boats the difference in performance between boats can be in the order of 50m in a kilometer - about 2%.  When you are truly in tune with the boat quite small differences feel quite large, for instance what is the difference between an old jib and a new one - just a couple of seasons use can make the sail feel "off".

Finally what difference to the mass is caused by humidity?  Heavy wind is often wet wind?

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