All you never knew about wind speed

Before visiting Oz, I always thought Ozzie windsurfers can plane in a lot faster in a given wind strength. Turned out the differences were not quite as big, since wind speeds were typically from hand-held meters on shore, while windspeeds reported in the US are mostly from meters mounted 10 meters up. That difference is a bit more than the 14% mentioned in the video. That said, I wish we'd have the smooth waters of Mandurah or Lake George somewhere here, and fast sailors like stroppo and slowie to learn from!

Thanks. That is a very well done explanation of a lot of factors.

Describing 'wind strength' is a VERY difficult thing.

And wind is NEVER "15 knots". There is always a dynamic range of variation. And that range varies greatly from place to place and in different physical environments as the narrator very aptly pointed out. And people individual interpretation of what "15 knots" means also varies widely.

Some models give an estimated gust strength and others do not.

What I have found is that if I observe the predictions of a certain model long enough, and compare it with what actually happens, I can often get a good feel for how to interpret that model for my spot in particular conditions.

Different type of Anemometers can colour your perception as well. Here is an example in a very unstable and gusty wind at Luderitz:

How would you call this wind strength?

Personally, I prefer the analogue anemometer in most situations as I can see instantly all the variation and get a feel for the variation. But there are some good digital anemometers than can do various user set averages as well.

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jn1 said..
Good link forceten, thanks . Interesting to learn about measurement inconsistencies.

I think a parameter relevant to our sport would be "Wind Power". This factors in "air density" (ie: the mass variable). 20kt in summer is not 20kt in winter.

We track it where we live. Winter 5C is different than summer, humid 35c plus we are at 350m altitude. In the winter time, we are effectively below sea level in air density and in the summer nearly 1400m.

Edit: should have added that the difference between winter and summer air density is over 10% for us - pretty significant in the light air summer months when we are struggling to plane. Our 7.5 just became a 6.7.

For the US (and some int'l locations), this is helpful thanks to the autoracing community: airdensityonline.com/tracks/

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Paducah said..

jn1 said..
Good link forceten, thanks . Interesting to learn about measurement inconsistencies.

I think a parameter relevant to our sport would be "Wind Power". This factors in "air density" (ie: the mass variable). 20kt in summer is not 20kt in winter.

We track it where we live. Winter 5C is different than summer, humid 35c plus we are at 350m altitude. In the winter time, we are effectively below sea level in air density and in the summer nearly 1400m.

Edit: should have added that the difference between winter and summer air density is over 10% for us - pretty significant in the light air summer months when we are struggling to plane. Our 7.5 just became a 6.7.

For the US (and some int'l locations), this is helpful thanks to the autoracing community: airdensityonline.com/tracks/

Here we go again! I can't help it .... No one has ever been able to explain to me how a 20kt wind measured in cool dense air has any more force vs thin dry hot air? These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

What a surprise, the USA doesn't follow the international convention.

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gorgesailor said..

Paducah said..

jn1 said..
Good link forceten, thanks . Interesting to learn about measurement inconsistencies.

I think a parameter relevant to our sport would be "Wind Power". This factors in "air density" (ie: the mass variable). 20kt in summer is not 20kt in winter.

We track it where we live. Winter 5C is different than summer, humid 35c plus we are at 350m altitude. In the winter time, we are effectively below sea level in air density and in the summer nearly 1400m.

Edit: should have added that the difference between winter and summer air density is over 10% for us - pretty significant in the light air summer months when we are struggling to plane. Our 7.5 just became a 6.7.

For the US (and some int'l locations), this is helpful thanks to the autoracing community: airdensityonline.com/tracks/

Here we go again! I can't help it .... No one has ever been able to explain to me how a 20kt wind measured in cool dense air has any more force vs thin dry hot air? These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

Here's how I understand it.

Say you have a feather and a brick travelling at 15knts each. They are both doing the same speed but the brick being more dense will hit you with more force.

So. 15 knots is 15 knots regardless of density. But as cooler air has more density it also has more force.

The confusion is the terminology used. We should be talking about wind energy and not wind speed.

To measure wind energy you need to know both temp/density and wind speed.

Select to expand quote

gorgesailor said..
These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

You're (mostly) wrong. Think of typical wind meter that has cups that catch the wind. The cups are mounted on a center axis with little friction, so they will rotate at the speed of the wind, regardless of air density. The device measures the rotation speed, not the forces exerted on the cups. One wind meter I have even use bike speedometers for that, since those are cheap to get.

At a given wind speed and humidity, colder air is denser. More air molecules will hit the cups of the wind meter, so the force they hit with will be higher. But the cups won't rotate faster than the wind speed because they hit air molecules on their way back which slows them down. I'd guess that they actually move marginally slower than wind speed, which is compensated for in the calibration.

When sailing in moderate climates, the difference in air density between cold and warm air is barely noticeable, partly because cold weather sailing usually makes us quite a few pounds heavier from wearing thicker wet suits (and boots and gloves). Also, keep in mind that the kinetic energy is linear to density, but increases by the square of the wind speed. So a 20% difference in density is more similar to a 10% difference in wind speed (until you take into account apparent wind and board speed, at least).

But the difference is very noticeable when you sail high elevation lakes like Lake Silvaplana (about 1800 m / 5900 m above ocean level). You may have similar wind strengths and temperatures compared to nearby Lake Garda, but you will need a sail that is one or two sizes larger, since air density is about 20% lower.

Select to expand quote

gorgesailor said..

Gestalt said..

gorgesailor said..

Paducah said..

jn1 said..
Good link forceten, thanks . Interesting to learn about measurement inconsistencies.

I think a parameter relevant to our sport would be "Wind Power". This factors in "air density" (ie: the mass variable). 20kt in summer is not 20kt in winter.

We track it where we live. Winter 5C is different than summer, humid 35c plus we are at 350m altitude. In the winter time, we are effectively below sea level in air density and in the summer nearly 1400m.

Edit: should have added that the difference between winter and summer air density is over 10% for us - pretty significant in the light air summer months when we are struggling to plane. Our 7.5 just became a 6.7.

For the US (and some int'l locations), this is helpful thanks to the autoracing community: airdensityonline.com/tracks/

Here we go again! I can't help it .... No one has ever been able to explain to me how a 20kt wind measured in cool dense air has any more force vs thin dry hot air? These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

Here's how I understand it.

Say you have a feather and a brick travelling at 15knts each. They are both doing the same speed but the brick being more dense will hit you with more force.

So. 15 knots is 15 knots regardless of density. But as cooler air has more density it also has more force.

The confusion is the terminology used. We should be talking about wind energy and not wind speed.

To measure wind energy you need to know both temp/density and wind speed.

Nope. What is being said here is that 20kt wind measured with a device has more force in dense air vs thin air. I say this is false since the device is already measuring the force & not the actual speed of the air.

Let's take the example of a common impeller driven anemometer. If the dense air has more energy when acting on your sail, won't it also have more force acting on the impeller?

Try again. Anyone?

Wind meters measure wind speed not energy. The density of the air has no effect on the meter.

You are correct that 15 knots is the same no matter the temp or density of air but the amount of force is what changes. So yes 15 knots in summer feels lighter than 15 knots in winter even though they are both 15 knot breezes

There is a bunch of theory on wind farms and how this relates.

Select to expand quote

gorgesailor said..

Gestalt said..

gorgesailor said..

Paducah said..

jn1 said..
Good link forceten, thanks . Interesting to learn about measurement inconsistencies.

I think a parameter relevant to our sport would be "Wind Power". This factors in "air density" (ie: the mass variable). 20kt in summer is not 20kt in winter.

We track it where we live. Winter 5C is different than summer, humid 35c plus we are at 350m altitude. In the winter time, we are effectively below sea level in air density and in the summer nearly 1400m.

Edit: should have added that the difference between winter and summer air density is over 10% for us - pretty significant in the light air summer months when we are struggling to plane. Our 7.5 just became a 6.7.

For the US (and some int'l locations), this is helpful thanks to the autoracing community: airdensityonline.com/tracks/

Here we go again! I can't help it .... No one has ever been able to explain to me how a 20kt wind measured in cool dense air has any more force vs thin dry hot air? These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

Here's how I understand it.

Say you have a feather and a brick travelling at 15knts each. They are both doing the same speed but the brick being more dense will hit you with more force.

So. 15 knots is 15 knots regardless of density. But as cooler air has more density it also has more force.

The confusion is the terminology used. We should be talking about wind energy and not wind speed.

To measure wind energy you need to know both temp/density and wind speed.

Nope. What is being said here is that 20kt wind measured with a device has more force in dense air vs thin air. I say this is false since the device is already measuring the force & not the actual speed of the air.

Let's take the example of a common impeller driven anemometer. If the dense air has more energy when acting on your sail, won't it also have more force acting on the impeller?

Try again. Anyone?

If you were to have 20 knots of wind flowing through a wind measuring device and then water flowing through the same device that was also flowing at 20 knots then the impeller would spin at the same speed. It's not going to spin the impeller faster than the water speed even though the water is obviously much more dense. We used a wind meter a few years ago in Maui that was also used in SE QLD. I can guarantee that 25 knots in Maui is a hell of a lot stronger than 25 knots is in SE QLD. The wind is very heavy and has lots of power. For me that's mega powered on a 4.5 looking to change down in Maui vs very comfy with the ability to handle more on a 4.7 over here.

I found this online. Its interesting as angle of attack comes into play. Prob why it's easier to sail upwind or downwind when overpowered.

Sail thrust is depend on:
*Plan form
*gap between foot and hull
*profile
*true wind speed
*angle of attack
*boat speed
*heel angle
*rig type /interaction between sails or other boats sailing nearby
*wind gustyness
*...

Basic formula :

F=C*A*(ro/2)(v^2)

where :
C -force coefficient
A -sail area
ro -air density
v -air speed across the sail

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qldnacra said..

gorgesailor said..

Gestalt said..

gorgesailor said..

Paducah said..

jn1 said..
Good link forceten, thanks . Interesting to learn about measurement inconsistencies.

I think a parameter relevant to our sport would be "Wind Power". This factors in "air density" (ie: the mass variable). 20kt in summer is not 20kt in winter.

We track it where we live. Winter 5C is different than summer, humid 35c plus we are at 350m altitude. In the winter time, we are effectively below sea level in air density and in the summer nearly 1400m.

Edit: should have added that the difference between winter and summer air density is over 10% for us - pretty significant in the light air summer months when we are struggling to plane. Our 7.5 just became a 6.7.

For the US (and some int'l locations), this is helpful thanks to the autoracing community: airdensityonline.com/tracks/

Here we go again! I can't help it .... No one has ever been able to explain to me how a 20kt wind measured in cool dense air has any more force vs thin dry hot air? These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

Here's how I understand it.

Say you have a feather and a brick travelling at 15knts each. They are both doing the same speed but the brick being more dense will hit you with more force.

So. 15 knots is 15 knots regardless of density. But as cooler air has more density it also has more force.

The confusion is the terminology used. We should be talking about wind energy and not wind speed.

To measure wind energy you need to know both temp/density and wind speed.

Nope. What is being said here is that 20kt wind measured with a device has more force in dense air vs thin air. I say this is false since the device is already measuring the force & not the actual speed of the air.

Let's take the example of a common impeller driven anemometer. If the dense air has more energy when acting on your sail, won't it also have more force acting on the impeller?

Try again. Anyone?

If you were to have 20 knots of wind flowing through a wind measuring device and then water flowing through the same device that was also flowing at 20 knots then the impeller would spin at the same speed. It's not going to spin the impeller faster than the water speed even though the water is obviously much more dense. We used a wind meter a few years ago in Maui that was also used in SE QLD. I can guarantee that 25 knots in Maui is a hell of a lot stronger than 25 knots is in SE QLD. The wind is very heavy and has lots of power. For me that's mega powered on a 4.5 looking to change down in Maui vs very comfy with the ability to handle more on a 4.7 over here.

Great analogy! And had the same experience in Maui.

I also think that another aspect of the wind can create a difference between what an anemometer with small cups shows and the force of the wind on the sail: the consistency or the micro-gusts of the wind.. If you blow into an anemometer, it shows some wind even after you have stopped. You could also blow in it intermittently and keep a relatively constant wind reading. My experience in Maui, maybe because of the venturi effect, the wind was relentless, compared to the gusty wind I sail in Elwood. A sustained airflow would generate more force on the sail but may not have the same level of impact on the speed of the small cups. Of course when you are sailing fast the apparent wind will reduce the effect of the gusts on the sail.

Select to expand quote

Gestalt said..

gorgesailor said..

boardsurfr said..

gorgesailor said..
These wind "speed" measuring devices are actually measuring the same mechanical forces as we feel on our sails. I understand the theoretical difference & factors as they relate to drag, but unless we are actually measuring the speed of the air molecules & comparing that to the forces they can exert, we can't really say a dense 20kt wind has more power than low density 20kt wind. Because the mechanical wind "speed" measuring device has already compensated for that.

Tell me I'm wrong & why!

You're (mostly) wrong. Think of typical wind meter that has cups that catch the wind. The cups are mounted on a center axis with little friction, so they will rotate at the speed of the wind, regardless of air density. The device measures the rotation speed, not the forces exerted on the cups. One wind meter I have even use bike speedometers for that, since those are cheap to get.

At a given wind speed and humidity, colder air is denser. More air molecules will hit the cups of the wind meter, so the force they hit with will be higher. But the cups won't rotate faster than the wind speed because they hit air molecules on their way back which slows them down. I'd guess that they actually move marginally slower than wind speed, which is compensated for in the calibration.

When sailing in moderate climates, the difference in air density between cold and warm air is barely noticeable, partly because cold weather sailing usually makes us quite a few pounds heavier from wearing thicker wet suits (and boots and gloves). Also, keep in mind that the kinetic energy is linear to density, but increases by the square of the wind speed. So a 20% difference in density is more similar to a 10% difference in wind speed (until you take into account apparent wind and board speed, at least).

But the difference is very noticeable when you sail high elevation lakes like Lake Silvaplana (about 1800 m / 5900 m above ocean level). You may have similar wind strengths and temperatures compared to nearby Lake Garda, but you will need a sail that is one or two sizes larger, since air density is about 20% lower.

Aha, much better! .... However I don't think I would say "mostly". I am not certain this is always true of cup style impellers but in any case, It seems logical enough. That said, to me you are oversimplifying this. After all what is causing the cups to rotate - the force exerted by the wind. What you are saying is that the cup is not efficient enough to overcome it's own drag & move faster than the wind. This could be true. But cup style impellers are not the only type of anemometer in common use. What about turbine style impellers? Are these not miniature foils being driven to rotate around a shaft by the force of the wind? Pitot tubes? Seems to me the only anemometer truly immune would be Ultrasonic, or Laser Doppler anemometers which measure actual air particle velocities.

I think qldnacra had an interesting analogy however once again not sure it fully applies... In any case, not doubting what we feel... just trying to understand WHY this has been so commonly accepted yet it seems to me the theory is at least partially flawed...

I think you missed the point. wind "speed" is not a measure of power or force. it's a measure of "speed". it doesn't matter at what particle level you measure speed. speed is not changed depending on particle size or density.

what you are trying to define is force not speed. force relies on the air density.

I don't think I missed the point. The POINT is that perhaps with a mechanical anemometer, when we think we are measuring speed, we are in fact measuring force & inferring speed. Whether or not this is accurate depends on the the specific mechanics. I suspect the cup style are less susceptible to variations in density due to the arguments presented here, but not so sure about the fan/turbine style. I have since learned that Pitot tube style anemometers should use a correction factor depending on air density.

When all is said & done what we really care about is wind force anyway so really just an intellectual exercise. I don't use an anemometer anyway I am so used to looking at the water state & other telltales. I can tell when I need my 3.7 whether I am in the cool moist wind on the Oregon Coast or in the 90F arid wind of the Eastern Columbia river Gorge...

Speed is not in fact what we should be worried about, nor air pressure, nor force nor power, in true maths terms it's the vector and the velocity that count.

In my opinion, you feel the direct one way force at your sail when your brain calculates the wind speed, but the device calculates the speed by difference of two counter forces;
For a 4 cup anemometer if you neglect 2 cups parallel to wind, there are 2 cups effected by wind force. One sides aerodynamic creates more force than the other side and creating torque on the center mil, which rotates it and we read the speed as how many rotations per a time interval.

As speed increases or density increases, force increases in about "squares" depending on area, so difference in force of two cups is increasing more and more. So companies probably use a standard producer to calibrate speed shown, which they take air density as constant.
When air density changes those calculations get higher or lower depending on cup aerodynamics.

Some high end devices may record air density as well and make a rational speed calculation.

At the end wind speed we read is a calculation made over forces that effect the device, not individual air molecule speeds.