There has been much debate about winter winds being stronger than summer winds despite the same air speed. Where is the proof in Layman's terms without reading long ass scientific explanations?

Here is my 2.3 cents worth (includes GST & Carbon Tax)....

I will try and explain in my own words without the need for complicated/scientific explanations...

Like everything air has a weight (specific weight) which is directly related to the Air Density. Let's not get too technical but colder air is heavier than warmer air (this is a proven fact). And this air is moving at a certain speed (eg 30 knots) and pushing your kite. The weight of the air has an impact on the pull that you feel. For example Imagine a 100KG weight with four lines connected to your harness and then propelled at 30 knots. It will generate a certain amount of pull versus your body weight (power to weight ratio). Now imagine a 50KG weight with four lines connected to your harness and then propelled at 30 knots as well. While you are gaining momentum the pull that you feel from the 50KG weight will be much less than the 100KG weight despite the weights being moved at the same speed and assuming that you do not vent any of the power. The power to weight ratio is different. Despite the difference in power to weight ratio you will end up at the same speed anyway but it will take longer using the 50KG weight.

In the real world the impact of air density (weight of the air) on kite pull is nowhere near as much as my example above but it does have some impact on what you feel from the kite. To be more specific (assuming a standard atmospheric pressure) the specific weight of air at 15C is 12.01 while at 30C it would be 11.43. So using my example above, the difference between 15C and 30C in 30 knots winds would be like using a 100KG weight versus a 95KG weight. The difference is about 5% and this will directly affect the power to weight ratio. Likewise, in a 45C climate the difference would be approx 9% (from 15C) and would explain the difference in kite sizes.

Of course there are many other factors involved but I hope this helps.

I would of thought the amount of moisture in the air would change the mass more.
ie; on a good seabreeze there can be so much moisture your kite never dries completely but on a cold easterly off the desert it dries straight away

Yes it does make a bit of difference but not very much.
Looking purely at aerodynamics it is density which is important not weight of air, also don't forget when you are working out the density you should be using K not C so add 273 to the temperatures. The density is a function of temp in K times pressure. So kiting in a winter low pressure system will reduce the density compared to a high pressure system.
The formula for lift also has density halved, further reducing the effect of temp changes, where as velocity is squared.
So the wind speed has the biggest effect of all.

let us use a couple of formula's do calculate that actual dynamic pressure the wind can applied to the kite.

Dynamic pressure is a defined property of a moving flow of gas or liquid and can be expressed as

pd = 1/2 ñ v2 (1)

pd = dynamic pressure (Pa)
ñ = density of fluid (kg/m3)
v = velocity (m/s)

Denisity can be calculated with the idea gas law.

p=p/(RT)
p=Pressure
r=individual gas constantj/kgdegk)
T=Absolute temp K


So. . . . .

16 deg density = 1.22kg/m2
32 deg desity = 1.15kg/m2

lets use 15m/s (30 knots) as a constant is our dynamic pressure calc.

dynamic pressure 16 deg = 137
dynamic pressure 32 deg = 130

Thats 5% difference.

So what else effects the density of the air?
The pressure and the humidity. Pressure is easy.An increase in pressure increases the density.

Humidity is somewhat more confusing. Water vapor is actually lighter than air. So the more humid the air the less dense the air is. .... that i can't be bothered calculating.

The end result is this. Because the pressure increase is 1/2 mass x velocity squared the impact of small changes in temp/pressure/humidity is small. The greatest impact is a change in velocity.

Nice one Plummet, your next challenge is to explain to KOTP why kites cant fly backwards using word of no more that 2 syllables.

beefarmer said...

Plummet said...

Humidity is somewhat more confusing. Water vapor is actually lighter than air. So the more humid the air the less dense the air is. .... that i can't be bothered calculating.

Hmm interesting thanks Plummet. I didn't know water vapor was lighter than air. This may explain why i was struggling so much in the fluffy sparrows fart warm moist air in Bali.

I always knew heavy dense air = more power (cold front vs summer sea breeze here in WA) and that moist tropical wind = gutless. But the warm bali air should be roughly the same temperature as a summer seabreeze here.... maybe a slight difference but i didn't think that alone would account for the complete lack off power over there. Higher moisture content and therefore lower density might explain the difference though.

Plummets calculations show its 5% +/- so virtually nothing, you were struggling because your kite was too small and the wind was too light. The difference is quite small 5% or even less, depending on the density of the air, not so much the temperature (i.e - is the wind in a low pressure weather system or high pressure system?)

If an experiment could be conducted, in different temp winds, the results would be inconclusive, because 5% difference (if it actually is that much, which I doubt) will be virtually undetectable by a kite flyer and the opinions of power would be subjective.

I'm sticking with my claim and advice that 30 knots somewhere is equal to 30 knots anywhere (which is actually a fact that can't be disputed)
The power that warm 30 knots in a high pressure system can deliver compared to 30 knots in a cooler low pressure system, is a very small bees dick of difference

cheers men.

i'll calculate humidty and pressure variation another day. my brain too has melted !

so a 16deg swing in temp eqautes to about 1 knot more or less of wind. at 30 knots +- 1knot probably isn't an issue. but at 12 knots +- 1 knot could be the difference in staying upwind or doing the walk of shame.

beefarmer said...

yet somehow, when you stand on the beach in bali, and you feel a good 25 kt wind in your face, and you look out and see the white caps a good 25 kt wind would produce, you rig up and go out and its not 25 kts, it's 18.

Or at least that's how it seemed to me.

Reckon thats more to do with local issues like depth of water, sun angles, and you being less familiar with wind in Bali - meaning you just under estimated the wind.
16 degrees of temp difference of the air can only make around 1 knot of difference (if that), not 7/8 knots like you are thinking.
Its got to be just underestimating.

Plummet said...

let us use a couple of formula's do calculate that actual dynamic pressure the wind can applied to the kite.

Dynamic pressure is a defined property of a moving flow of gas or liquid and can be expressed as

pd = 1/2 ñ v2 (1)

pd = dynamic pressure (Pa)
ñ = density of fluid (kg/m3)
v = velocity (m/s)

Denisity can be calculated with the idea gas law.

p=p/(RT)
p=Pressure
r=individual gas constantj/kgdegk)
T=Absolute temp K


So. . . . .

16 deg density = 1.22kg/m2
32 deg desity = 1.15kg/m2

lets use 15m/s (30 knots) as a constant is our dynamic pressure calc.

dynamic pressure 16 deg = 137
dynamic pressure 32 deg = 130

Thats 5% difference.

So what else effects the density of the air?
The pressure and the humidity. Pressure is easy.An increase in pressure increases the density.

Humidity is somewhat more confusing. Water vapor is actually lighter than air. So the more humid the air the less dense the air is. .... that i can't be bothered calculating.

The end result is this. Because the pressure increase is 1/2 mass x velocity squared the impact of small changes in temp/pressure/humidity is small. The greatest impact is a change in velocity.

This is the point in my earlier post (simplified). The weight of the air is directly related to the air density and the power that we feel from the kite. The specific weight of air at 16C is 12.01 while at 30C it is 11.43 (assuming a standard atmospheric pressure). The difference in the air weight is 5%, likewise in Plummet's calculations. There are other factors involved but the effects are minimal.

The difference in power that you feel from the kite at 16C and 30C is approx 5%. The difference between 16C and 45C is approx 9% (Plummet please check my math!).

At the end of the day, if the wind was 30 knots I would still use my 9M in winter and summer regardless of the air density (or air weight). At worse (like in winter) it would feel like 33 knots versus 30 knots in summer.

ok 16-45 deg yeilds a 10.5% drop in force. .

just fiddled some numbers on pressure change going from 1013hpa to 980 hpa which is a fairly typical high v low pressure change.

That yeilds a 5.5% change in force also. The higher pressure obviously increasing the force.

So if we had a 16 deg Hi pressure and compare that to 32 deg low pressure then we have a 10.5% change in force.

Next up humidity calculation.... himm i need to think about this abit. its getting complicated.

just to clarify i calculated that 16 deg hi pressure would be 10.5 greater force that 32 deg low pressure. of course you can get hi and low pressure systems in winter and summer.

If you have low pressure winter wind 16 deg c and 980hpa and 32 deg 1013hpa summer comparison your roughly at the same power. . . . . . . . . . . . . . . . .

This is all based on dry air calcs. i'm yet to do hummid air calcs.

AGK said...

I'm sure the density of air has something to do with it but coming from a sailing background I have had quite a few coaching sessions with top Olympic coaches as well as weather experts,

my understanding is a thermal wind as opposed to a gradient wind generally will be stronger especially in aus. Take Perth for instance huge land mass in summer to heat up not far from the colder water of the coast, this means a huge amount of air rising over the land sucking in the cold doctor(aka seabreeze) same thing in NSW. If you notice on a summer morning likely to be a light offshore wind blowing this is the cooler air filtering out to the ocean, watch for the clouds on the horizon lift and start to make way to the ocean meaning the seabreeze won't be far away. This is a thermal breeze, now in winter the land doesn't get warmer than the sea temp often usually the opposite this means the gradient wind will stay in all day and as the two temps level out we generally get that ****ty nothing winter wind.

Places like Bali and smaller islands don't have the land mass to generate this effect so if it's close to a large land mass it may be effected by its thermal wind but generally its winds will 100% be driven by way of pressure system movements. Ie clockwise around a low pressure system and anti-clockwise around a high pressure system( in the southern hemi. ) that and it's local trade winds.

That's why it will feel windier closer to shore (def in summer) aswell, the further you move away from the coast generally the less the wind will be due to that temp difference. again depending on fronts.

I concur.
Thermal wind, such as sold WA seabreezes, are consistently strong because of the significant temperature difference between the land, and its hot rising air, and the cold sea air which rushes in to replace it.
Whereas, frontal winter weather can be A LOT stronger, but only in gusts before and during the passing of the front.
I was told the front is area between two competing weather systems, like two blokes in the boxing ring, so gusts and wind direction is highly variable - think a southerly buster on Sydney Harbour after a 42C day in summer.

Kitepower Australia said...

beefarmer said...

yet somehow, when you stand on the beach in bali, and you feel a good 25 kt wind in your face, and you look out and see the white caps a good 25 kt wind would produce, you rig up and go out and its not 25 kts, it's 18.

Or at least that's how it seemed to me.

Reckon thats more to do with local issues like depth of water, sun angles, and you being less familiar with wind in Bali - meaning you just under estimated the wind.
16 degrees of temp difference of the air can only make around 1 knot of difference (if that), not 7/8 knots like you are thinking.
Its got to be just underestimating.

I'm sitting in the airport at Bali ATM about to fly home after two weeks here, I have to say I totally agree with Beefarmer it always feels so much stronger than it turns out to be.
I bought over a 9,12 & 15, never used the 9, used the the 12 a couple of times but was mainly on the 15, yet standing on the beach and looking at the whitecaps I could have sworn I would be ok on the 9 most of the time.

cheers guys for doing some of the highschool maths + physics I've long since forgotten how to use. very interesting discussion, i was convinced from my experiences that there'd be no debate at all, that someone would soon enough find an easy explanation.

not so, but i don't think I've been proven wrong just yet, just that so far we haven't found the right explanation.

Has anyone else out there got similar (or totally conflicting) observations of warm moist tropical locations being gutless compared to thermal seabreezes (that is, gutless in terms of power in the kite, compared to how windy you expect it to be when you are standing on the beach?)

Or is it just me when i'm in bali?

I believe that air density has an influence on the amount of power one feels in a kite.
The most well documented or obvious example of this is how altitude effects air density.
30 knots of wind in Johannesburg (at 6000 feet) feels a lot less powerful than 30 knots of wind in Cape Town(at sea level). This is because the air density is roughly half in Joburg than at sea level.
I do believe that the example that plummet used where all base on sea level and that at sea level the variations in density can be as great as 10 to 12% depending on local pressure, temp and humidity. I feel that this power difference is perceivable. I feel that a 5% difference is perceivable.

yep all my calcs are at sea level.

The difference between sea level and 6000ft all other aspects being the same is a 18% drop in force.