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 Post subject: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 9:44 am 
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Hi guys, I have a doubt. When I read some topics I find a very different evaluation about kite range.
I know from saling studies that the "density" of the air can influence the "strongness" of the wind, so at the same wind velocity there is a different range of the same kite if you are in equator or in less hot countries (where air density is minor).
Range kite reported in official brand website ... Where is referred to?


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 10:17 am 
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Joined: Wed Oct 16, 2002 1:00 am
Posts: 5555
Location: Denmark
alessio roma wrote:
Hi guys, I have a doubt. When I read some topics I find a very different evaluation about kite range.
I know from saling studies that the "density" of the air can influence the "strongness" of the wind, so at the same wind velocity there is a different range of the same kite if you are in equator or in less hot countries (where air density is minor).
Range kite reported in official brand website ... Where is referred to?


You are right, there are difference in "power" with different temperatures indeed :thumb:

But regarding windcharts it does not matter at all, as they are so personal biased and only a very very rough estimation, based often on different non clarified grounds.
In other words: Windchart/range charts are so different and can no be compared, so the difference the air temperature gives, is negletable compared.

You could say that many range charts are either "off", or at least not useable when you dont know the given conditions.
That being said, they are still a fine guide to find approximately where the kite has its sweetspot with a given weight rider and board :naughty:

You could see this old thread: http://www.kiteforum.com/viewtopic.php?f=1&t=5159&start=10

Or this one: http://www.kiteforum.com/viewtopic.php?f=1&t=2343149

I would estimate most charts to be in the neighbourhood 25C / 75F if one should have a guess.

8) Peter


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 10:44 am 
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30 degree change in temp is about a 10% change in force.


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 3:52 pm 
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Air temp is one factor... the other is water temp..it is their differential that makes it happen or not

Charts may be created with 75F air and 75F water.....and a 75kg rider :wink:

In Florida when air is warm from southerly direction over cold water 10 kn can have power of 14+kn

But a colder northerly wind direction over warmer water 10 kn will have a 7-8kn power :(

So put a kite up and go on the water because on land it will be different for the power in the kite

Besides temperatures i have found that the wind direction angle in relation to the coastline...
...is also a factor that affects power on the water

These are observations where i kite only and may not be the same at other latitudes :cheers:


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 8:28 pm 
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Here's the calculation for calculating the density of air, which is dependent on
Temperature (colder = more density)
Air Pressure (varies by frontal systems, and altitude)
Humidity (more humid means air is less dense, weird but true)

http://en.wikipedia.org/wiki/Density_of_air

There used to be a weather site that calculated boost factor based the wikipedia calculations. It compared air density at sea-level on a 20 degree day with 50% humidity to current conditions world wide.

Someone posted about this a few years ago.
http://www.thechickenloop.com/index.php?option=com_fireboard&Itemid=99&func=view&id=7455&view=flat&catid=2
Google "Kite Density Boost".


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 9:14 pm 
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Location: The Naki
SupaEZ wrote:
Air temp is one factor... the other is water temp..it is their differential that makes it happen or not

Charts may be created with 75F air and 75F water.....and a 75kg rider :wink:

In Florida when air is warm from southerly direction over cold water 10 kn can have power of 14+kn

But a colder northerly wind direction over warmer water 10 kn will have a 7-8kn power :(

So put a kite up and go on the water because on land it will be different for the power in the kite

Besides temperatures i have found that the wind direction angle in relation to the coastline...
...is also a factor that affects power on the water

These are observations where i kite only and may not be the same at other latitudes :cheers:


I hate to dispel your myth easy. But the density of water changes only very marginally with temperature change. A 20 degree increase in water temp yields a .4% decrease in density. The reality is that water temp is not going to affect the power in the kite.

Except for when there is a water/land temp differential. Then you will get a convection style sea breeze drift by the temp differential.

You will probably find that its current that is yielding the difference at you local. Not water temp. In one wind direction It will be wind against current and with the other it will be current with wind.


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 10:42 pm 
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Location: Ford Lake, Michigan
supa has it, charts are for 175 pound (about 80kg) riders sea level 75F (about 24C) standard air pressure.

vapor has less than 1% effect on air density you can ignore it, hi and low pressure systems are also less than 1%

the air pressure according to elevation is a bit complicated but its almost linear near the ground so it can be approximated 10k/(10k - altitude), temperature that obeys the gas laws so you just measure in K and divide.


for example, you are 85 kilograms, riding at 500 meters in temperature of 15C, what do you multiply the wind chart times?

weight: well you weigh more so that will make the wind chart higher because you are bigger, that factor is 85/80 about 105%

altitude: the altitude is 10k/(10k-500) or about 105% you need a bit more wind there

temperature: it's colder so you need less wind here remember 0c is 273K so convert and multiply 288/297 or about 97%

so you need 105% times 105% times 97% or just about 105% of the wind chart

for the example case, 85 k riding at 500 meters in 15C if the chart says your wind range is 11 to 26 knots instead multiply by that factor 1.05 and you get a wind range of 12 to 27 knots

There you go! Not much difference in the wind range is there for my example.

Things to pay attention to here are if you are kiting in extreme cold, or if you are a very large person, over 250 pounds or 115 kilograms, or very tiny less than 50 kilos or if you are kiting a mile or two above sea level.

Otherwise the charts are close enough. You can do the exact calculations if you like.


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 Post subject: Re: Air density and wind velocity
PostPosted: Sun Apr 27, 2014 11:27 pm 
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Joined: Wed May 21, 2003 12:07 pm
Posts: 1397
Location: NW Pacific
Breaking bad..
wind.
Depending
on the power,
of the shower,

can be horrible.

H2O temps can mightily
amplify,
the power of the cower
you will be in
when it hits your head.

The differential
of your preferential
depends on the density
of your propensity
for profligation.

Rule of thumb
being.......
Pass the gas
before
enter-
ing.


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 Post subject: Re: Air density and wind velocity
PostPosted: Mon Apr 28, 2014 7:47 am 
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[quote="plummet"]
I hate to dispel your myth easy. But the density of water changes only very marginally with temperature change.[quote]

Don't think he was talking about density of water, just that water temp affects wind speed.


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 Post subject: Re: Air density and wind velocity
PostPosted: Mon Apr 28, 2014 8:50 am 
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Location: The Naki
sijandy wrote:
plummet wrote:
I hate to dispel your myth easy. But the density of water changes only very marginally with temperature change.
Quote:

Don't think he was talking about density of water, just that water temp affects wind speed.



Its land to water temp differences driving sea breezes. Not water to air temp.

Apart from convection style breezes created by temperature differentials on land/water I simply can't see how water temp will affect the wind power all other factors being equal.


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