I hope you can settle a bet between me and a coworker. With it having rained earlier today, it was quite humid, and my coworker said "The air is thick and heavy today." I tried to explain to them that, contrary to popular belief, the air is actually lighter when it is more humid. Isn't that true?

MIKE MOSS SAYS:       Ryan,     So long as you specify a couple of necessary conditions to make for a fair comparison, you are correct. Assuming all else is the same (no difference in pressure, temperature, volume of air considered, etc) humid air is less dense than dry air. This is because the molecular weight of water vapor is less than the average molecular weight of dry air, some of which is displaced when water vapor is mixed in, or when the concentration of water vapor increases. I suppose it also depends on how one defines "thick and heavy." At first blush, it would seem to be a pretty good equivalent to "dense," but there may also be a perceptual issue here in terms of how we humans respond to increased humidity and the "sticky" or "muggy" sensation that high humidity levels produce, regardless of the actual air density.