Humidity in the air has a small effect on the oxygen content,
when then has an effect on horsepower output from the engine.
A much larger effect is the bouyancy of humid air relative to dry air.
Because humid air is less dense than dry air, it tends to rise.

A small vertical velocity of the air mass will give a large change in
the cruise speed of slippery little lancairs. 100 fpm lift or sink is
a hardly noticeable pitch change if you push or pull to maintain altitude.
But that amount of lift or sink could easily add or subtract a few kts
from cruise speed. How much speed change do you see if you
push the nose over to give a 100 fpm descent?

To take an extreme case, 1000 fpm lift under a line of cumulus
is enough to allow some sailplanes to maintain well over 100 kts
with zero engine horsepower. In still air, the same aircraft cannot
maintain altitude at any speed.

So... if you measure a speed difference that is correlated with humidity,
the speed difference is probably caused by vertical motion in the airmass,
rather than horsepower changes.

-bob mackey
 (no engine for my first 2000 hours :-)

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