James McGinn on these forums has advanced the conjecture, that liquid water evaporates not as a monomolecular gas, as customarily assumed, but in the form of "microdroplets" - liquid clusters of two or more water molecules held in cohesion by hydrogen bonding. Deriving directly from this is the hypothesis that moist air is heavier than dry air, again contrary to the customary assumption that moist air is lighter: Conventional science reasons that evaporated water forms a gas of single water molecules which mix with the other gases in air; that the molecular weight of H2O is less than that of the primary air constituents O2 and N2; that therefore the addition of water vapor makes the air less dense. McGinn proposes in contrast that microdroplets of N water molecules will behave as larger molecules with molecular weight N times the molecular weight of water; that these heavier pseudo-molecules will therefore make moist air heavier than air devoid of water. Or so I take the liberty of paraphrasing McGinn's ideas.
McGinn has stated in support of the microdroplet conjecture, that the high level of intermolecular hydrogen bonding in liquid water creates the circumstance that actually it requires more energy to remove a single water molecule from the surface of liquid water, than that required to remove a cluster of two or more water molecules, in which the hydrogen bonding would be partially "neutralized". Now I will say, that I found there to be a certain seductive attraction to this revolutionary idea - could it really be true? I have no idea quantitatively how much energy it takes to remove a molecule or a cluster of them from a liquid surface. But the concept has a certain ring to it hinting at the possibility of an undiscovered truth. So it appeals to me according to my ways of thinking, similarly to other diverse non-mainstream notions: vast cosmic electrical currents in space; the world trade centers destroyed by controlled demolition; the Cheops Pyramid as not a tomb but some type of Machine built according to principles lost to modern science; etcetera.
But even more poignantly, I felt the question of the density of moist air to be of critical and direct importance to my own life: For years upon years, when my shoes were wet, I would lay them down to dry them: Because it always "seemed to me" that, well, of course the humid air inside the shoes would be denser than the surrounding air, and so sort of "drain out" faster that way, with the shoes lying down. I did the same always with wet gloves, balancing them with the fingers pointing up. I trained my kids to do this as well, educating them ad nauseum on the best way to position gloves and shoes for proper drying. (My wife resisted such training.)
But then, again reading on this very forum, I was informed by Mr. Charles Chandler that in fact, according to modern science, humid air is less dense and will rise. Well I realized I had never actually thought about this... but, it seemed to be correct, and all my years of upside down shoes nothing but foolishness. Then again, now here was McGinn, arguing that actually the moist air was heavier, and would drain best downwards from my shoes and gloves.
I was much troubled by this question, and resolved to determine the true answer. But it is not easy to make a direct and accurate measurement of the difference in density of moist versus dry air. You can try to create some rigid container, and weigh it containing dry and then moist air. But just some rough calculations will show that this requires making uncommonly precise measurements of weight, in addition to other practical difficulties. Similar problems arise in any related methods.
Long I considered how to contrive some clear and valid experiment; finally the following method occurred to me:
I took two clean glass bottles, and wet them thoroughly inside, drying their outsides. I suspended them both upside down for several hours, to allow any dripping water to drain out. This left in each bottle, a random assortment of remaining water droplets, adhering to the inner walls. Then I inverted one of the bottles - so that now one was suspended upside down with the neck facing down, the other upright with the neck facing upwards.
Now, I reasoned, if the humid air is less dense, it will rise out of the upright bottle, and that one will dry first. If the humid air is more dense, it will drain out of the upside down bottle, and _that_ one will dry first. In either case the humid air will be trapped in the contrary bottle, and delay its drying.
Some complications to consider:
-- There is the possibility that water continues to drain out of the inverted bottle. This would assist the drying of the inverted bottle, and therefore favor the "heavy humid air" hypothesis. Therefore I shook as much water as possible out of both bottles at the beginning of the experiment, and made sure to let them both drain upside down together for several hours. But still it would be a consideration to keep in mind.
-- Evaporation will cool both bottles. Again this will favor the drying of the upside-down bottle and the "heavy humid air" hypothesis. Something else to keep in mind.
The typical result of a trial of this experiment, I illustrate with the following image:
The full resolution image, by the way, I've uploaded to my website, so it's accesible here:
As shown, there is a lot of moisture remaining in the inverted bottle, whereas the upright bottle is fully and completely free of any visible water.
So it seems that humid air is less dense than dry air after all.
In this case, the bottles were left in the shown position overnight. This is a room that I keep dehumidified and dust free with air filters, so it's quite climate controlled. However for the duration of the experiment all air conditioning, heating, fans, and filters were turned off, so the air in the room was still apart from normal thermal convection.
Actually I tried this several times: Sometimes with the bottles positioned so their necks were at the same level; sometimes positioned so the bodies of the bottles were at the same level; sometimes switching which bottle was inverted, in case somehow one bottle had more mineral deposits on the inner walls, or otherwise retained more adhered water than the other for some reason.
But always the result was the same - in the morning the upright bottle was dry; the inverted bottle was still wet.
Henceforth I shall leave my shoes upright to dry, and try to educate my children to do likewise.