wind density

juandesooka · Post by **juandesooka** » Tue Aug 27, 2013 11:50 am

As a relatively new wind chaser, I still find wind dynamics a mystery. Here's an article that explains why 20kts at cook st may not be the same as 20kts at nitinat:
http://www.progression.me/news/2013/08/ ... ted-equal/

Along the same lines, I find it fascinating that 30kts is not twice as strong as 15kts.....as I understand it, it's a geometric progression, multiples stronger. Much like ocean waves, more than just height, also width and depth ... have to think in 3 dimensions.

Cue the twilight zone music, makes my head spin!

downwind dave · Post by **downwind dave** » Tue Aug 27, 2013 1:22 pm

the example compares cold winds at south africa to warm winds in brazil. I don't think you get that range of air density fluctuation between Cook St and Nitinat. but you never know!

KayakDoc · Post by **KayakDoc** » Tue Aug 27, 2013 2:38 pm

This discussion invites a nice blend of qualitative and quantitative approaches. Where and how the wind speed is measured is a factor. Experience with the local waters is another. The great thing about wind sports is coupling local wind/tide predictions/recordings to my experience to maximize my fun on the water. All the facts and none of the flavour would be pretty dull.

rocdoc · Post by **rocdoc** » Tue Aug 27, 2013 4:19 pm

This topic sounds like a lot of hot air to me.
More relevant to sport ballonists that kiters.

firstonlastoff · Post by **firstonlastoff** » Tue Aug 27, 2013 4:54 pm

Wind power density is dependent on the cube of the wind velocity (that is the speed (use m/s) of the wind raised to the power of 3). Hence, 15 knots generates significantly less wind power than does 20 knots of wind.
- However, the temperature dictates the density of the air. As the air density increases from approximately 1.18 kg/m3 at 25 degrees centigrade ( eg. like Nitnat without a fog bank) to 1.27 kg/m3 at 5 degrees centigrade (eg. anywhere in Vancouver Island or lower mainland area in December or January and beyond), the wind power increases linearly by the ratio of 1.27 divided by 1.18 = 1.07. So wind density might make 20 knot winds at 25 degrees centigrade feel like 21.5 knots at 5 degrees centigrade - certainly a bit more power, but not like you'd be heading back to the beach for a smaller foil.
- I think the wind quality (average, lulls, gusts, temporal duration of each), such as gust strength and duration may in fact be more important. In winter sailing/kiting when a gust that is from a frontal system is sustained over tens of seconds, that is more important than air density because of the cubic factor on wind speed and hence feel like you should have been on a smaller foil, whereas the Nitnat summer 'gusts' appear as mere blips on the sail/kite (wind quality at Nitnat is outstanding), and as has been said before, it can feel like you're cutting the grass.

windaddiction · Post by **windaddiction** » Tue Aug 27, 2013 7:50 pm

As far as 30 knots being way more powerful than 15knots, is due to the difference in kinetic energy.

Kinetic energy = 1/2mass x velocity squared

So all things being equal in the mass of the air, every time the velocity doubles, the kinetic energy quadruples!

Makes those redbull storm chaser guys seem insane!!!

Atomic-Chomik · Post by **Atomic-Chomik** » Sun Sep 08, 2013 10:40 pm

thankgodiatepastafobreaky · Sun Sep 08, 2013 11:30 pm

Right Dave
Kinetic energy = 1/2 mass x velocity squared ...

but temperature affects the mass of air so the denser (colder air) will have a higher mass and conversely warmer air will have less mass.
Colder air has a higher density and so more kinetic energy or force on the sail. We need a 9M in the Straight of Juan de Fuca when they need a 12M in Aruba with the same windspeed.

I always thought that as geeky wind and weather observers who are endlessly looking for better tools to assist in choosing the best sail size, why not come up with a new wind scale that has temperature (and moisture for that matter) worked into it.

MartyD · Post by **MartyD** » Mon Sep 09, 2013 9:05 am

While we are at it we need to build that number into the sails and kites too!
It can be a factor or projected area, kite size, and max turning speed, ect!
Science!
Pasta you get to name the unit!!!!

thankgodiatepastafobreaky · Mon Sep 09, 2013 9:11 am

how about the DREADKNOT

Dreadknot = windforce on sail = Kinetic energy = 1/2 air mass (including temp, humidity) x velocity squared

..anyone know how to include the temp and humidity properly??

Then we also need a sail/kite performance value scale that can include the apparent wind which you are talking about Marty.

juandesooka · Post by **juandesooka** » Mon Sep 09, 2013 9:21 am

Dude....I am loving this. If you come up with a new wind measurement that takes off, and it actually becomes known as the DreadKnot (DKt), I will be [smilie=beer.gif]

thankgodiatepastafobreaky · Mon Sep 09, 2013 9:26 am

ok ... we need Ross now...

..and Ross replied..."don't get me started"

JL · Post by JL » Mon Sep 09, 2013 9:57 am

I've been thinking about this for a few years. I need to try an extended trip with warm wind to build a personal data base to make a qualitative assessment of warm winds.

Purely in the interest of science.

Moderate Wind Mateo · Mon Sep 09, 2013 10:08 am

... and to complicate things don't forget 1) wind doesn't blow it sucks and 2) wind doesn't flow it a straight line it veers and clocks goes up and down and wheels like a high speed fairiswheel.

mortontoemike · Post by **mortontoemike** » Mon Sep 09, 2013 2:35 pm

I just did a back of an envelope calculation and for a 20 kt wind (10.28 m/s) the force is only 5% less at 25 C relative to 10 C and 7% less at 30 C relative to 10 C.

I will get around to calculating the collision frequency for a 6 m^2 sail at some point.

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