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difference in the strength of cold/dry vs warm/humid wind

Posted: Wed Jan 16, 2013 7:08 pm
by coleman
I was out on two consecutive light days. one from the south which was warm and relatively moist air from the gulf, the next day was a light north wind. both days seemed to be in the 10-16mph range and i was on a 15m edge.

i got upwind great on the south wind but really struggled on the north. Is there a difference in these air currents based on moisture and temp alone?

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 7:44 pm
by plummet
cold = more dense = more power
dry = more dense = more power
wet = less dense = less power
hot = less dense = less power.

but not by much. you need significant changes in humidity and temperature to make any notable difference. i've worked it out before and can't remember the exact results. but it was something like a 5% increase/decrease from 0 to 30 deg C.

The most likely cause was current in the water. that plays a bigger effect.

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 7:59 pm
by letsgoflyakite
In uk, so grey and cold. When I went to Greece, the sea was white capping everywhere, so up went my 9. Wrong ! Gutless. Came in and put up a 12, which was fine. If the sea looked like that in UK, a 12m would have punished you badly. So hot and dry in Greece = less power than a grey and cool uk (summer as we like to call it) . So your right, windspeed isn't everything. Current will of course have an effect, but in Greece current is negligable. Buy one size kite and travel !

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 8:04 pm
by SSK
There definitively is, but as everyone pointed outer there is sometimes a lot more to it. The force of air is directly proportional to the density of the air. Although we say "it is so humid I could cut the air with a knife", in truth water vapor is less dense than air. So humid air is less dense. Hot air is less dense then cold air. This is why aircraft have a hard time taking off in hot, humid, high altitude airports.
If you ever kite Texas on a hot summer day then head out to the Central California coast (cool, dry, dense) you will likely notice that you are overpowered with the exact same amount of wind on the same kite.
Now there is a lot more going on, especially because what you describe should be contrary to what you would expect. First wind is never constant, it is like a sine wave with peaks and valleys (gusts and lulls). The tighter these are the better. In South Texas with warm water and land the Northerly winds are usually frontal where the Southeast is more thermal. I think the Southeast is more consistent in Texas (less peaks and valleys). But this consistency trade winds still seem very strong even though they are hot and humid. Also in South Texas the predominant wind is from the South causing some Southerly current. The N winds are frontal and do not usually last as long. So there is often a northerly flowing current for a while after the front passing. Although contrary to what you describe, sometimes you have doming in South Texas in the Spring where the air is warm but the water is still cold and the cold inversion causes boundary layer seperation. So it may be reporting 15, but from the surface up to kite height it may be much lower.
So the answer is yes, but there are a lot of other factors which could make the results seem converse as you have pointed out.

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 8:15 pm
by Miltsface
plummet, I'm with you on the cold vs warm, but I would think that moist would be more dense than dry. is that just one of those things that's counterintuitive? or maybe warmer air can hold more moisture, so it really comes back to cold vs warm?

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 8:27 pm
by SSK
Yes, it is counter intuitive because of the way it feels to your body. Moist air does not support evaporation of sweat making you feel hotter, and it is harder to breathe. So it feels uncomfortable and thus associated with being heavier. But here is a good explanation.
The amount of water vapor in the air also effects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

The two most abundant elements in the troposphere are Oxygen and Nitrogen. Oxygen has an 16 atomic unit mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the troposphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.

You may be familiar with the concept that moist air is less dense than dry air. This is true when both have the same temperature or when the moist air is warmer. Said in another way, air with a greater percentage of water vapor will be less dense than air with a lesser percentage of water vapor at the same temperature. Often people erroneously believe that moist air is denser than dry air because very moist air is more difficult to breathe than dry air.

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 8:29 pm
by coleman
thanks for the good info, must be that particular lake which is known for being crappy on the north probably due to houses on the north shore causing the air currents to be turbulent. thought i would ask though as i knew there was a difference in the relative strength but couldnt quite remember how it works

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 8:33 pm
by edt
PV=nRT, P ~ NT, so for instance at 25c, humid air density is 118.38, dry air density is 118.43, so you get about 0.04% more power from completely dry air than completely wet air (not much) and if the temperature ranges from 25 to 15c (298 to 288k) you also get about 3% more power from each 10c colder air (makes your 12m feel like a 12.4m). A hi pressure system might have 1023mbar more than 1013mbar average pressure and will give you about 1% more pressure on the kite. Of course high pressure systems typically have low wind speeds so not sure this matters. Denver by the way might have a pressure of 840 or so, or 20% less power in your kite in Denver. Very substantial. Something to think about if you take your kites on a snow kiting trip.

Temperature is more important than humidity. And altitude is more important than temperature. Temperature will give you about 1 meter worth of kite for every 20c change (about 40F change).

I think people have worked this calculations out a dozen times in this forum. the formula is PV=nRT. Also remember that even when it's 100% humidity that still means you still have less than 1% water vapor in the air.

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Wed Jan 16, 2013 8:47 pm
by Kiteus Maximus
I kited 2 days back to back where the 1st day had winds blowing 15 kts with a relative humidity of 90% and the next day blowing the same 15 kts after a cold front moved in and the relative humidity dropped to 20% and the power difference was night and day even though the wind speeds were the same.

The cold day with low humidity felt more like winds at 20+.

Re: difference in the strength of cold/dry vs warm/humid win

Posted: Thu Jan 17, 2013 3:36 pm
by Bille
I Really notice that difference whenever i go from Mohave
to the coast in San Diego !

A 25 in the Dry Mohave desert, isn't anywhere Near as strong
as that coastal air with the same wind strength. I remember Mohave
being about 640ft above sea level, so it isn't really an altitude thing.