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From: Gary Casey <glcasey@adelphia.net>
Subject: Humidity effects

Scott posted an interesting question about humidity.  The fraction of  
air that is actually water can get to be significant at high  
temperatures and it has two effects.  It reduces the power of the  
engine because less of the air is oxygen.  It also reduces the  
density of the air (less lift, ASI compensation difference), but in a  
different proportion(not that my data contradicts this statement).   
This is because the molecular weight of water is less than air - 18  
compared to 29 or 30 for dry air.  At 100F air at 100% relative  
humidity is  about 7% less dense than dry air.  At 70F the difference  
is only 2.5% and at 40 degrees it is only 0.8% different.  Multiply  
those numbers times the actual relative humidity to get the density  
difference at any given condition.  Regarding the power reduction of  
the engine it depends on the fraction of air that is water and the  
numbers at sea level are as follows:  At 40 degrees the partial  
pressure of water is 0.121 for 0.8% oxygen (or power) loss.  At 70 it  
is 2.5% and at 100 it is 6.5%.  I'm running out of time this morning,  
but note that according to the quick look in my tables, the change in  
density appears to be the same as the change in water fraction and  
that shouldn't be the case.  I'll look into it a little more.  In any  
event, the difference is small at low temperatures, but it goes up  
directly as the density of the dry air goes down - if you are at an  
altitude where the density is half that at sea level the effects are  
twice what I show above.  Of course then the temperatures are usually  
lower.  I don't think that could explain the differences in airspeed  
that Scott saw - however there are occasionally large areas of  
lifting air that could result in that sort of thing.

Gary Casey