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