Huh, interesting. Thanks.

Quote Originally Posted by hvacrmedic View Post
The water vapor displaces other heavier molecules, reducing the overall density of the gas mixture. Just imagine filling a room with helium (at constant pressure). As the helium content rises the density of the mixture in the room will drop, and the room will gain lift like a helium balloon.

Less dense air will be poorer at transferring heat from an air over heat exchanger like a finned coil. Aircraft will also have less lift. A helicopter can land on a high peak in dry air, and be unable to get back off the ground later due to higher humidity alone. It has happened, or so I've heard.