[paul cotter (OP)]

Ten minutes ago I heard a statement from a meteorologist, on BBC, that "dry air is very dense". That is a sweeping generality that may or may not be correct, depending on circumstance.

[alancalverd]

In these islands, it is generally true. And the BBC has little interest in weather further afield.

At a given temperature and pressure, dry air is denser than noncondensing humid air. Dry air mostly reaches the British Isles from the arctic high, either directly (cold) or via the Continent (warm)  and is associated with high pressure. Wet air mostly comes from the Atlantic low and is associated with low pressure. So at a given ambient temperature, in general if the humidity is low the density will be high. 

[paul cotter (OP)]

That is not sufficiently pedantic for me. He could have said cold dry air from the arctic is dense etc,etc. But to say dry air is dense without further qualification is just not a scientifically rigorous statement. You will be familiar with the "hot and high" problem in aeronautics where low density is a problem in areas with predominately low humidity.

[alancalverd]

Fact remains that any given P and T, dry air is denser. I doubt that the BBC meteorologist was ignorant and/or addressing aviators at the time. Where it matters, your met brief will include dewpoint but it all becomes irrelevant above 1000 ft AGL anyway.

[paul cotter (OP)]

Okay Alan, it's one of those mea culpa moments, again. I guess I didn't think it through: o2 weighs ~32 and n2 weighs ~28 while h2o comes in at ~18 and simple displacement will lead to lower ρ.

[Petrochemicals]

In that case Paul why does h20 not rise through n2? Hydrogen does.

[paul cotter (OP)]

I can counter that with another question: if hydrogen rises, then why does xenon not sink to the floor?  Hydrogen en masse will rise, like in a balloon, but if you were to slowly release hydrogen into a sealed room my guess is that it would slowly diffuse to reach an even concentration throughout the room. Gas atoms and molecules are constantly colliding at fairly high speeds and this in my opinion will lead to even mixing. Not really my field and I could well be wrong.

[Petrochemicals]

I can counter that with another question: if hydrogen rises, then why does xenon not sink to the floor?  Hydrogen en masse will rise, like in a balloon, but if you were to slowly release hydrogen into a sealed room my guess is that it would slowly diffuse to reach an even concentration throughout the room. Gas atoms and molecules are constantly colliding at fairly high speeds and this in my opinion will lead to even mixing. Not really my field and I could well be wrong.

Argon does infact sink through air, not so sure about xenon. Helium is known for disappearing into space. Hydrogen may have some property of being in a molecule of h2.

[alancalverd]

Xenon sinks. And if the temperature is above dewpoint, H2O rises. The situation is complicated by the numerous temporary polymers of water at low temperatures, which slows down the diffusion of water through the atmosphere as the "effective molecular weight" depends on concentration and temperature. But if you wait long enough, you end up with a dry planet like Mars where only the CO2 is retained by gravity.

[Bored chemist]

Ten minutes ago I heard a statement from a meteorologist, on BBC, that "dry air is very dense". That is a sweeping generality that may or may not be correct, depending on circumstance.

The molecular weight of water (18) is quite a bit less than that of oxygen (32) or nitrogen (28).
So, all other things being equal, dry air is denser than wet air.
The use of "very" is pushing it.

[Bored chemist]

f you were to slowly release hydrogen into a sealed room my guess is that it would slowly diffuse to reach an even concentration throughout the room.

Not quite.

Let's start by thinking about the air. The pressure falls with altitude (the effect inside a room is tiny, and usually swamped by temperature variations etc).
The rate of change with altitude is called the lapse rate and it is dependent on things like local gravity, temperature and the molecular weight of the air.
And, if you had a room full of hydrogen, the pressure at the ceiling would be less than that at the floor.
And, again, the lapse rate would depend on the molecular weight etc.
So it would be different from that for air. (But still tiny)

The interesting thing is that the hydrogen does not "know about" the air.
So, at equilibrium, the concentrations of the gases settle down so as if the other gas is not there  (I'm pretending that "air" is just one gas for simplicity.)
And because the lapse rates are slightly different, the concentrations (i.e. pressures) change with height in different ways.
So the ratio of the gases change with height.
(I'd not like to have to measure the effect)

[Bored chemist]

Xenon sinks. And if the temperature is above dewpoint, H2O rises. The situation is complicated by the numerous temporary polymers of water at low temperatures, which slows down the diffusion of water through the atmosphere as the "effective molecular weight" depends on concentration and temperature. But if you wait long enough, you end up with a dry planet like Mars where only the CO2 is retained by gravity.

Unless your planet is big, like Jupiter or cold like Europa.

[alancalverd]

So the ratio of the gases change with height.
(I'd not like to have to measure the effect)

We do have good data for the ratios of "well behaved" gases like N, O, Ar and CO2 (even Rn) with altitude and they are fairly consistent, particularly above the tropopause.

But H2O is not well behaved in Earth conditions as it exists in all three phases and various intermediate polymers, at all altitudes and with significant and continuously evolving differences at different times and positions - conventionally called "weather".

However what the meteorologist said is generally true enough over the British Isles  for the purposes for which the statement was made.

[Bored chemist]

if you were to slowly release hydrogen into a sealed room

...
You would not get what is...

conventionally called "weather".

[alancalverd]

Probably because H2 does not exist

in all three phases and various intermediate polymers, at all altitudes and with significant and continuously evolving differences at different times and positions

in Earth ambient conditions.

But if you stand in a really big sealed room, like an airship shed, the H2O does indeed make weather when the sun shines!