> Can this be right?
> Air laden with water is lighter than air?

Yep, that's right.

(I doubt if this point will be argued to the point
of name-calling, so I will dare to post a message
and attempt to explain.)

First, we need a cubic foot container.  Mine are much
nicer than the usual Tupperware stuff, as they weigh
absolutely nothing, and are perfectly clear.  Expensive,
but well worth it.  (The cubic meter ones cost even more!)

OK, so we fill two containers with "dry air".
But what's really in "dry air"?

If there is NO water vapor in the containers, they have about
78% nitrogen molecules, each having an atomic weight of 28.
About 21% of the air is oxygen molecules, with an atomic
weight of 32. (Let's just ignore the other 1%, as it is a
mix of other semi-random gasses, and will just complicate things.)

Now these cubic foot containers are vented, so individual
molecules can move in and out.

If we add water vapor molecules to one of our cubic feet of air,
some of the nitrogen and oxygen molecules would be pushed out.
Avogadro discovered long ago (1800s?) that the total number of
molecules in our cubic foot of air must stay the same if we do
not change the temperature or pressure. (Newton knew this also,
but did not know [or never explained] why.  Avogadro explained
the mechanics, so he gets credit for the "discovery".)

But what's water's atomic weight?  18!  Much lower than either
nitrogen or oxygen.

So as we add water, the water molecules that replace nitrogen or
oxygen are lighter than either the nitrogen or oxygen.  Even a
small difference in humidity can result in "lighter air".

  "But wait a second", you think - "water CONTAINS oxygen!
  How the heck can water be LIGHTER than oxygen?"

  Yes, water does contain oxygen, but only one atom.
  Oxygen gas is a molecule with two oxygen atoms.
  Each oxygen atom has an atomic weight of 16, and
  a hydrogen atom has an atomic weight of 1.

  H2O  is water.  2 Hydrogens and 1 oxygen.  2 + 16 = 18
  O2 is oxygen gas.  2 Oxygens.              2 * 16 = 32

So, when we put our containers of "dry" and "humid" air on a
balance, the "humid" air weighs less.

In technical terms, replacing some of the nitrogen and oxygen with
water vapor decreases the density (and thereby, weight) of the same
cubic foot of air.

But does this really matter all that much in a beehive?  Not really.
Temperature matters much much much much more.

The effects of temperature and barometric pressure on air are much
much greater than the effect of humidity on air density, but humid
air >>IS<< lighter than dry air at the same temperature and pressure.

Offhand, I'd guess that "water vapor effects" really only matter in
beehives that are overwintered in locations were bees would not survive
without things like hive-wrapping, places where the air temperature is
so low that the air warmed by the bees is cooled to the ambient
temperature before it gets anywhere near the top of the hive.

In most places where bees are kept, when you combine the effect of the
exhalations of the bees (which are warm), the heat mass of a bee cluster
(which has to warm at least some of the air in the immediate area), and
the water vapor from the bee exhalations, you have air that is sure to
"rise", or at least not sink to the ground.

That's why inner covers can get damp in unvented hives here in
Virginia to the point where the cheap ones made with masonite
will actually warp or "bulge".

        jim (A recognized expert on "hot air")

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