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WIND STRENGTH MYTH
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onehullcat
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Sep 21, 2011, 7:13 AM

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Well, 'knot' is a measure of wind speed, not wind force...


The Publisher
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Sep 21, 2011, 4:28 PM

Post #2 of 12 (24669 views)
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TRUTH IS...
Wind strength myth: In colder weather 20 knots of wind is stronger than 20 knots of wind in warmer weather. True or false? The answer is...




WIND STRENGTH MYTH
This factoid regarding wind strength and air temperature comes from Ari Barshi of the Laser Training Center in Cabarete, Dominican Republic:

"I actually had a NASA rocket scientist help me on this one. His conclusion was: As the air is denser in colder weather, and assuming you hold everything else constant (i.e. sail size and sea conditions, as changes in the roughness of the water will change the boundary layer and affect the actual wind pressure on the sail), a change in temp of 5 degrees C will bring about a 2% change in total wind energy. So 20 knots of wind in 8 degrees C does feel like just 19 knots in 25 degrees C."

More here: http://www.sailingscuttlebutt.com/news/11/0920/


The Publisher
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Sep 21, 2011, 4:28 PM

Post #3 of 12 (24668 views)
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The answer about wind speeds at different temperatures generating different amounts of force is incomplete. Yes, the colder air is denser so will generate more force at a given windspeed, but the enquiring mind will realise it doesn’t end there. What are you using to measure that wind speed? If the measuring device is affected by air density as much as the sail is, then the density effect is already accounted for. There's also the question of water density/temperature changes ( sail forces must be balanced by hydrodynamic forces), and the effects of viscosity change with temperature…….

Kim Klaka


The Publisher
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Sep 21, 2011, 4:28 PM

Post #4 of 12 (24667 views)
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I am not a rocket scientist (and I can give you references on that), but assuming the NASA scientist that Ari Barshi consulted when determining that in colder weather 20 knots of wind is stronger than 20 knots of wind in warmer weather ('Butt 3431) is correct about the density-temperature thing, I have a question: how would you know it was blowing twenty knots? Wouldn't the difference in air density effect the performance of any instrument used to measure the wind velocity? I used to have this conversation with Don Barrus, an old Lido 14 friend (and an old friend of the Curmudgeon) relative to wind strength on Big Bear Lake versus wind strength on Mission Bay: there were those who believed the absolute strength of the wind at five-thousand feet was less than the same wind at sea level, but we wondered how the anemometer would know at what altitude it was. I think more research on this is owing. Grant money, anyone?

Chris Ericksen




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Sep 21, 2011, 4:29 PM

Post #5 of 12 (24664 views)
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Am I reading this incorrect or should the numbers be 19 knots at 8°C feel the same as 20 knots at 25°C.

David Foscarini
Scarborough, Ontario Canada


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Sep 21, 2011, 4:29 PM

Post #6 of 12 (24663 views)
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A quick comment on the wind strength myth. Cooler air is denser, but as one who sails with a wing, I have to say that different winds have different feels. There is no better way to feel the wind than with a Kitewing http://www.kitewingusa.com/product-overview

You literally hold the wind in your hands. Some winds, whilst cold feel braided and fraught with voids. Humidity affects the "pull" coming from the wind and some of the best and most "punchy" winds come in spring when the air is moist and flowing like water.

William Tuthill




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Sep 21, 2011, 4:30 PM

Post #7 of 12 (24661 views)
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The impression left by the "Wind strength Myth" answer is I feel misleading as at higher temperatures you need a higher velocity to feel the same force, not lower! My comment is:

Psychologically a 20 knot wind in March, when it is COLD feels much more daunting that it does in balmy August temperatures, but that may be due to the contrast between an anticipated capsize into cold water lumbered with a dry suit and clothing, compared to a pleasant, if slow, swim in shorts in the summer.

At fixed pressure, the density of a gas is inversely proportional to the Kelvin temperature, i.e. Centigrade plus 273. The force on the sail is proportional to the rate of change of the momentum of the air, which in turn is proportional to the velocity squared times the density. For the same force on the sail, with the only change being the temperature of the air, the equivalent velocity is proportional to the square root of the Kelvin temperature. Then for a 5 degree centigrade change the equivalent to a 20 knot wind at 20 C (293 K) is a 20.17 knot wind at 25 C (298 K). It would take a change from 0 C (Iceboating) to 28 C to produce the quoted change from 20 knots to 21 knots (not 19 knots). The humidity also affects the air density and can change with temperature, so my conclusions strictly apply to dry air.

Peter Hinrichsen

- Craig Leweck, Scuttlebutt


onehullcat
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Sep 21, 2011, 5:52 PM

Post #8 of 12 (24649 views)
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Let me try again: a knot is a measure of speed not force, so if both a Beneteau 473 (~35,000 pounds) and a Squadron Yacht Minuteman (500 lbs) were approaching you at twenty knots, do you think the force of impact is the same for each on your Lazer, or would you choose?


Mal
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Sep 22, 2011, 8:39 PM

Post #9 of 12 (24546 views)
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Wind speeds are the equivalent of airspeed in flying. Indicated airspeed is a measure of the force of the wind. I would assume that most all fixed wind speed measuring devices measure this speed. Excluding some small errors on a standard day at sea level, it will also be the measure of the true wind speed.

As temperature or altitude increases or barometric pressure falls (density reduced) this measurement becomes less accurate (lower). It is then best labeled as indicated wind speed. Indicated wind speed is easily corrected for these variables to get true wind speed. It still is indicated wind speed that best measures affects on the sails not true wind speed.

To illustrate; a simple hand held anemometer held in a breeze in Denver would indicate a wind speed less than the speed a balloon or smoke would travel when released but would be a better indication of how the boat would react.

That colder air at a given true wind speed exerts more pressure on the sails, all else equal, is certainly no myth.
Check Six .......Mal


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Sep 23, 2011, 7:50 AM

Post #10 of 12 (24520 views)
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From Ari Barshi:
Thanks for the inclusion of my Wind Strength Myth in Scuttlebutt 3431, but I'm afraid I made a mistake.

To back up, I began to search for the difference of the wind pressure in dense air, as my racing results in cold weather are clearly worse compared with racing in warm weather which is what I am used to in the Dominican Republic. The important conclusion is that although there is a difference, our finding of 2% for every 5 degrees C change is minor and insignificant, assuming everything else stays the same.

But when reporting the facts in my newsletter (and the inclusion of this info in Scuttlebutt), I have reversed by mistake the data. I should have written that "20 knots of wind in 25 degrees C feels like 19 knots in 8 degrees C".

My goal was to get to a formula similar to the "wind chill factor" used in weather forecasts. Maybe in the future we will hear, "Today the wind will blow 22 knots with a wind feel factor of 26 knots".


From Brooks Magruder (Istanbul):
Regarding density of wind/air, Tuthill (in Scuttlebutt 3432) mentions "punchy" wind when air is moist. Actually dry air is denser than humid air at same temperature/pressure because water molecule H2O is lighter than DISPLACED nitrogen molecules N2. BTW, salt water is denser than fresh water because salt doesn't need to displace any water in same volume.



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Sep 26, 2011, 6:44 AM

Post #11 of 12 (24469 views)
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From Derek Paterson:
Regarding wind strength at different temperatures, without being scientific it is the density of the air, not the speed that makes the difference. Compare being hit by a marshmallow at 20 knots with being hit by a same size piece of solid chocolate at 20 knots. The marshmallow would be a surprise - the chocolate would hurt.


Mal
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Sep 26, 2011, 6:59 AM

Post #12 of 12 (24468 views)
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Ari, what the wind "feels like" is not the same as how it affects the sails of a boat. While the effects of temperature are indeed minor and your first post had them going in the wrong direction. How wind affects sails is directly proportional to its density. Air density is indirectly proportional to temperature. Therefore as the temperature decreases, density increases and the effect on the sails increases.

Use of the word "feel" evokes a human reaction to the wind. Humans react more to variations in heat transfer and evaporation when determining how a wind "feels". Was that dark chocolate Derek?

Since all individual sailing contests take place in an area and time of the same altitude, temperature and humidity; and variations in wind speed on the course are huge in their effects compared to those other variations anyway; the inclusion of variations in the way sails are affected by differences in temperature is totally meaningless at any temperature a contest is likely to be held in.

Altitude, however, has a more noticeable effect in density at plausible sailing locations and thus how the wind affects the sails. Denver sailors are aware of this factor but, I suspect use wind measuring instruments not calibrated for that change in altitude so their indicated wind speed is very close to the same effect their sails would react to at that measured wind speed at sea level. I doubt they even know that if they released a balloon it would travel a bit further than their indications of wind speed would predict ...... nor do they need to.

Now, as to the next AC45 regatta at the Lake Titicaca Yacht Club ...............
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Check Six .......Mal




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