Humid air is less dense than dry air. Effect of humidity is more pronounced in higher temperatures. At 20°C, 99% RH humid air is about 1% less dense than dry air. At 30°C about 1.5%, 40°C about 2.7%. A 3°C, 4.5°C and 8°C rise in air temperature has roughly the same effect respectively.
I remembered reading somewhere that those take-off performance charts (FAA) assumes 50% RH condition for temperature up to 30°C, reducing linearly for higher temperatures. (But I don't know the logical reasoning behind this)
Like you, I am also interested to know more.
[This message has been edited by Old Dog (edited 30 June 2000).]