Does the wind have more force on a yacht's sail in humid conditions, as in the tropics, because of the higher water content of the air?
Just read the same question on the back pages on New Scientist, I assume you picked it up from there too.
Made an interesting read 
Air density would be lower, but the moisture in the air may serve to reduce permeability in a sail reducing the amount of pressure lost through the sail.
Computer says too many variables, not enough information. ;)
I'm going to have to read that New Scientist bit now. I think i've got the last one sitting at home.
Without readingthe answer I will take a guess, I am a bit bored at work.
I would say more (my guess). With the air being more dense it should have more energy related to the momentum it has (p=m(ass)v(elocity)) so more energy gets transferred to the kite making the reaction to the wind, more.
I'll stand in the corner and shut up if I am wrong 
Water vapour is less dense than air. You'd think it would be more because water is heavy, but it's actually less due to the molecular weights of H20 vs N2.
Water molecules are just extra objects for the wind / air to move, so there is less force on the sail. Cool air is denser, and as long as it is dry it should be the most forceful on the sail.
At the end, can you flash up the appropriate answer from the New Scientist....I am too lazy to look....
Thanks
Jquigley posts some interesting thinking there. I think a correction that needs to be made is that for the surface area, you need to use projected area (i.e. a 12m kite has a projected area of less than 12m).
Another way to measure the force would be to put a loadcell between the spreader bar hook and the chicken loop so that you can get the instantaneous force at a point in time. With a suitably designed setup, you could work out the force vectors. Add in an accelerometer attached to the kite and you can link the the force on the chicken loop to the movement of the kite.
An engineering student took a Flysurfer (if I remember correctly) and did a bunch of wind tunnel tests on it; the results may only be relevant for foils, but it would be interesting to see the outcome.
Maybe there is a Seabreeze user out there who has some CFD skills and a bit of free time on their hands who might be willing to whip up a quick model to see what's what 
There's so many variables here that it's pretty much only ever going to be a theoretical exercise/stab in the dark.
I like the idea of putting a load cell at the chicken loop. Add to that a GPS so vertical and horizontal velocity and acceleration can be measured, have them both synched and put the data into some whizbang engineering software and see exactly what forces are at work - or in this case at play...
Are there any brainy computer technophiles out there?
What's CFD? Contracts for difference? Computational Fluid Dynamics?
I give up, my brain hurts.
J-P
If you're after a rough guide to what forces can be provided by a kite, at the apex of a jump (rider acceleration = 0) the upward force of the kite will equal the weight (f=ma) of the rider (and his board etc). So an 80kg rider will need an uplifting force of 80(kg)*9.8(m/s/s)= about 800Newtons. Or 80kg up
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