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Wind Shear: AnotherReason for Inflatable Superiority

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Wind Shear: AnotherReason for Inflatable Superiority

Postby Pump me up » Sun Jul 27, 2003 11:47 pm

Wind shear!

Wind shear is the change in wind direction and/or speed with height. We can also have horizontal and well as vertical shear, but it is the vertical shear that is of most interest to kitesurfers.

How does wind shear occur? It all comes down to good old friction, the closer you are to the surface of the sea or land, the slower the wind speed and the more the wind direction will be veered (southern hemisphere) in relation to the gradient or the friction-free wind. This change in wind speed can be easily detected between the kite overhead and the kite low-down and even though the direction of the true wind changes ever so little in the lowest 30 metres, it's the direction of the relative wind or the wind felt by you and by the kite that can change quite markedly between the sea-level and the 27m apex overhead. It all comes down to the stability nature of the air-sea(land) interface. Differences in speed range from very little in unstable air (around 5%) to enormous amounts in stable air (up to 300% ). From a directional point of view, differences range from about 1 degree in unstable air to about 30 degrees in stable air.

It is wind shear and not changes to the air density that causes what is known as the "weight of the wind". Even though the air density will change with a change in the air temperature, the change in the actual value of the air density will be very small. It is the variation in wind speed and hence the wind shear between the apex and the water surface that is largely responsible for this "weight" phenomenon.

In stable air there will appear to be more "weight in the wind" since there will be generally a stronger wind speed at the apex than at sea level. In unstable air, there is generally good mixing or overturning taking place in the boundary layer so the wind speed is fairly constant between the apex and sea-level.

What's all this stability stuff? In a nutshell it all comes down to the temperature difference between the sea and the air sitting on it! If the temperature of the water is colder than the air then we have a stable layer. On the other hand, if the sea temperature is warmer than the air temperature then we have an unstable layer. When the sea temperature equals the air temperature then we have a neutral layer.

Apart from temperature considerations, wind shear will generally be associated with a developing sea breeze situation. It can be very marked whilst this breeze is building and will drop away to nothing once the breeze has reached maturity. Shear can also be marked when one is close to a towering cumulus or cumulonimbus (thunderstorm) cloud and also if a cold front is close by.

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Postby BLOWN AWAY » Sun Jul 27, 2003 11:58 pm

Awesome stuff PMU.

But what's that gotta do with Inflatables being better than Ram Air kites?

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Postby sq225917 » Mon Jul 28, 2003 12:02 am

yet another fabulous insight into the working of pmu's paste and copy button.

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Postby Pump me up » Mon Jul 28, 2003 12:13 am

Wing tips!

Wing tips harness the effects of wind shear.

Far from being a disadvantage (as ram lovers would claim), the C or U-shape of an inflatable is an ADVANTAGE. Rather than being redundant, the semi-rigid tips of an inflato harness wind shear and facilitate a kite's dynamic, versatile qualities.

Yeah, I know there are some ram airs with a C-shape too, but the vast majority have a flat shape (thanks to the bridle). eg the whole lineup from Flysurfer.

Imagine the kite out at 45 degrees: With wind shear, there is more wind acting on the upper tip than there is acting on the lower tip. This means that with an inflatable, wind-shear has the following advantages:
- Acclerates the kite when sending it (making jumps higher)
- Helps to stabilise the kite at its zenith (auto-zenith feature!)
- Makes it harder to drop the kite accidently (kite wants to keep flying upwards)
- Speeds up turning and facilitates sining, making the kite feel lighter on the bar
- Facilitates kite loops

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Postby chemosavi » Mon Jul 28, 2003 12:47 am

Some ancient buddist philosophers would say that the more you think about your enemy the more you become like him. Be careful PMU, you might have a dream where you are flying a ram air and it beats you up and then you wind up in bed together. Secular kite dynamics. Christian or Jewish. Shiite or moslem. My girlfriend is prettier than yours. Variety is the spice of life.

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Postby zimboflyman » Mon Jul 28, 2003 12:47 am

I reckon you've forgotton to bring into play the apparent wind generated by the kites speed through the air!
Once the kite startes sineing and moving through the air, it creates it's own wind and i would say that this would pretty much nullify any tiny amount of wind speed difference between the top and bottom wing tips due to wind shear, when the kite is at the mentioned 45 degree angle.
I agree that wind sheer is a major influence in the wind speed between surface level and 30m up at the kite zenith, but to reckon that it can make a difference for the few metres between wing tips????
I doubt it!

The info on actual wind sheer was pretty cool though!

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Postby Pump me up » Mon Jul 28, 2003 3:34 am

I've been out in ultra-steady wind where there was amazing wind sheer happening. At the apex (27m) there would easily have been 30 knots. At water level, there would have been 5 knots, if that. So that works out to about 1 knot for every metre. The kite span from tip to tip in the air is about 7m. This means a gradient of 7 knots or so between tips. That IS significant! But even a few knots is probably significant.

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Postby BLOWN AWAY » Mon Jul 28, 2003 5:01 am

Cool.... you can choose how strong a wind you want by the height you fly your kite... :thumb:

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Postby theflyingtinman » Mon Jul 28, 2003 6:33 am

BLOWN AWAY wrote:Cool.... you can choose how strong a wind you want by the height you fly your kite... :thumb:
Yeah, even your direction - that's how ballooners do it. But of course they are little more educated on wind shear (have to be) and know that it's not a linear function of altitude, especially within 30m of the surface.

Steve T.

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Postby KiteGlider » Mon Jul 28, 2003 7:49 am

Pump me up wrote:I've been out in ultra-steady wind where there was amazing wind sheer happening. At the apex (27m) there would easily have been 30 knots. At water level, there would have been 5 knots, if that. So that works out to about 1 knot for every metre. The kite span from tip to tip in the air is about 7m. This means a gradient of 7 knots or so between tips. That IS significant! But even a few knots is probably significant.
Yes, good stuff, huge wind sheer! That place would drive me nuts.
Now my 16m2 flat foil has a wider wing span, so while on it's side the upper wing tip will get the higher wind power. That makes my foil MUCH more superior of course. :D

The water temp thing is interesting. I notice a difference with the kite's power when crossing from land to water, seems to loose power. I only notice this in 8-9 mph. From 10mph and up I have enough power so the temp change makes no difference.

Your area has interesting weather conditions.
NW Florida sucks now. Thunder storms almost everyday. Strangest summer so far.
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