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Name: Tiago
Status: student
Grade: other
Country: Portugal
Date: Winter 2011-2012


Question:
Hello, in the local kite board forum we are discussing how flying in a 'high' evaporation environment and the correspondent air humidity coming from will affect the lift of the kite. There are two side of the discussion. Side A's rational is that density is hardly affected by humidity content and consequently so thus lift. The back-ground for this is the following: A) water molecular weight is lower than dry air molecular weight and consequently a mixture of both gases will have less density (for a given volume, at same pressure and temperature). B) Advogardo's Law - for any given gaseous mixture the number of particles is constant within a specific volume, if pressure and temperature being kept constant.

Side B is claiming that as water, by either the action of temperature of pressure, moves from liquid to vapour then those molecules will need to go somewhere. As those molecules will need to go somewhere then the overall weight is higher and so is density.

My question is: When in an evaporation environment where those water-vapour particles go in order to kept the Avogadro's Law? I wonder if the atmospheric volume grows to compensate those water-vapour particles being added to the system, resulting in higher atmospheric weight (pressure) but with less density than the previous "non evaporation" state? But to be fairy honest I am a bit lost in my own rational at this point.



Replies:
It is true that the pressure in the immediate vicinity of the water source will be higher than ambient, because water molecules are continuously being added. But this just means that the volume of air in that vicinity will be expanding, pushing out and up until at some height above the water source (inches, maybe?) its pressure is the same as that of the dry air around it. At that point, the humid air will have a lower density than the dry air, and it will continue to rise for that reason. The density difference will be small, however, because only around 1% of humid air (50% relative humidity at room temterature) is water.

The lower density will decrease the lift you would have gotten from dry air in the same state of motion, but the humid air will be rising, which will increase the lift. I cannot guess which effect would be larger, but I suspect condensation of water onto the kite surface (if it occurs) could easily swamp them both.

Tim Mooney


At a fixed temperature, the body is buoyed up by a force equal to the mass of the volume of the amount of displaced fluid. So "in principle" lighter air is less buoyant. However, this effect is minor because a kite has very little volume to displace the fluid (air -- wet or dry). A far more important influence is the wind speed and the angle that the kite makes to the direction of the impinging air. Your "kite board" is confusing the behavior of a "kite" and a "balloon". The dynamics of the two devices is governed by different factors. No sarcasm intended, but I could noesist, "Go fly a kite!".

Vince Calder



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