We all like bone dry days because it helps our stacks of wood get dry. So I have a question for you about humidity and air. What is more dense, as in heavier per a given amount of volume. The most humid air Houston or New Orleans can offer on a hot summer day or the driest of dry summer air that can be found in a place like Vegas or Phoenix or the Mohave desert? The qualifier is that it is at the same temperature. Hint: The answer is found on the table that was up on the wall in your high school science classroom.
I'm not sure I understand what your asking. 80* and 90% humidity or 30% humidity, a cubic yard of each, which one weighs more? The 30% weighs more because water vapor is lighter than the usual gases that make up air. More water vapor in humid air means less gases.
Lessee... airplanes have more lift and thrust in dense air. Not so much in wet humid air. Water molecules and air molecules share the same cubic space. Assuming that humid air shoves dry air molecules further apart, so less dense air when humid. Dry air more dense... ?
Right, although it might feel heavy and oppressive, humid air is less dense at the same temp and pressure. Water is H2O, air is about 99% O2 and N2. If you look at the periodic table from chemistry class, hydrogen's atomic weight http://www.outdoorhub.com/news/2014/07/08/video-man-uses-forwarder-rescue-bear/is 1, oxygen is 16 and nitrogen is 14. (And molybdenum is 96). So a water molecule is going to add up to 18, oxygen is 32 and nitrogen is 28. When a water molecule turns to gas it has to shove something out of the way. So the gassed water dilutes the O2/N2 in your air making it less dense.
