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Name: Tiago
Status: student
Grade: other
Country: Portugal
Date: Spring 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:
Tiago,

First, Avogadro's Law is for an ideal gas at a fixed temperature, pressure, and volume. The real atmosphere does not maintain this number of molecules per volume, therefore

If water vapor molecules exactly displace the same number of dry air molecules, the density of the air will decrease. Normally, in the real atmosphere, air density increases as water vapor is added from evaporation from Earth's surface (and the air expands slightly) because the atmosphere does not expand quickly enough to maintain Avogadro's Law.

So, increased humidity can lead to increased air density and more lift for the kite. But that increase in density is so small that any lift advantage should not be noticeable.

David R. Cook Meteorologist Climate Research Section Environmental Science Division Argonne National Laboratory



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