Originally Posted by 7rider
Please explain why humid air is less dense than dry air.
Humid air would have more water molecules in it - making it more dense.
The density we're referring to here the ratio of air molecules to total molecules in the volume being measured. I believe the effect is that the water molecules don't behave quite the same as the air molecules, changing the viscous effects and kinetic behavior of the atmosphere. This may not be rigorously technically correct, and I'll defer to someone with more knowledge than I...
I've always thought that humidity's effect on density altitude had more to do with power production from IC engines, where the higher ratio of water molecules and lower ratio of air molecules in a given volume reduce the amount of combustible air available to the engine and thus reduce the power output.