Moist Air Less Dense than Dry Air

[tiggermoth]

I'm studying for the Met exam. Just read the statement that "Moist Air Less Dense than Dry Air".

I don't understand how adding a water molecule makes it less dense.

Help !

[Cobalt]

:8 says:

Water molecule mass: H2O = 2 x 1u + 16u = 18u
Nitrogen molecule mass: N2 = 2 x 14u = 28u
Oxygen molecule mass: O2 = 2 x 16u = 32 u
Average air molecule: ~29 u

so by adding lighter water molecules you reduce the average molecule mass and hence the air density (mass per volume)

C.

[DFC]

Get a class container of 1 cubic metre.

With only 1 cubic metre of air filling it, you will find that the air will under ISA have a mass of 1.225Kg. - density 1.225Kg per cubic metre.

If you then partly fill the same container with water, the air will be displaced and there will be less air in the container. Thus the mass of air is reduced and taking the container as a whole, the density has reduced.

Think only of how much air is left when moisture is added because when it comes to for example operating a piston engine, it is the amount of air per cubic metre that counts.

Regards,

DFC

[Floppy Link]

i.e
water molecules don't count towards the air density measurement - if air molecules are displaced by water molecules then density goes down.

[Saab Dastard]

Hmmm,

Don't forget temperature effects - DFC alluded to it by referring to "under ISA", but no harm to explicitly state that Moist Air Less Dense than Dry Air at the same temperature.

Cold, moist air can be more dense than hot, dry air. This is how water injection of turbojets works, by spraying cold water into the compressed air to cool it and increase its density, therefore allowing a greater fuel burn and resultant thrust. :8

SD

[bencoulthard]

LOL call me a simpleton but DFC made more sense to me than cobalt!!

That's just mathematical nonsense!!!!!!!!

[Crash one]

I did that one a couple of weeks ago & I had the same trouble, but it's the water "molecule" that is the answer, it's not a "droplet" but the gas, H2 O1.
Because of the 2 parts hydrogen it is lighter than an air molecule.
They are talking about a volume not a quantity. so the volume / box of cold air is more dense with more molecules of the heavy stuff, if you warm it up it expands & some of it escapes from the box leaving room for something else / hydrogen/oxygen. Although the oxygen is the heaviest of the three there are less of them.
That's why clouds of water molecules rise (except in up here in Scotland where they drape themselves over hills & sprawl over 07 threshhold!

Took me several thinks to figure it out.

Good luck with it.

[DenhamPPL]

I'm studying for the Met exam. Just read the statement that "Moist Air Less Dense than Dry Air".

I don't understand how adding a water molecule makes it less dense.

Help !

Don't worry tiggermoth I had EXACTLY the same thought a few weeks ago when I was working through the Confuser. In fact so confused did I get I decided to do the Nav exam first!

DFC's answer has cleared up the confusion for me. Hope he did for you to!

Andy

[tiggermoth]

Thanks all for your replies. It makes much more sense now. :)

The thing that really confused me was the thought that I was imagining that the air had the capacity to hold a certain amount of water in the same way as a sponge does. A moist sponge weighs more than a dry sponge.

The other thing that confused me too was that I was not thinking of the water molecule in the gaseous state, but I was thinking of it as a water droplet. Water droplets fall out of the sky as they are heavier than air and form rain (or is it only when temperature is below dew point? Eeek - that's a point, how on earth do clouds stay up there?!!). :ugh:

[WynSock]

In my humble opinion, DFC is one tool short of a full kit. That was the worst explanation I have heard for a while. The density of water is 1000kg/m3.

Cobalt spelt it out correctly. Water vapour is simply less dense than air.

So the more vapour, the less dense the whole parcel.


sheeeesh

[Microfright]

I think the important fact to help understand this is :-

“For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume” (taken from Wikipedia)

This means when comparing the same volume of dry air with moist air, the moist air has had some of the heavier molecules that make up air removed and replaced with lighter water molecules. Hence the same volume of moist air is lighter.

In terms of aircraft performance this has two effects.

1.The less dense moist air provides less lift.
2.The moist air contains fewer oxygen molecules (due to them being replaced by water molecules) and engine power is reduced.

[mad_bear]

Water vapour (i.e., water in the gaseous state) is lighter than air. A mixture of, say, 50% air to 50% water vapour is therefore lighter than 100% air. So it rises.

The problem comes when people think in terms of `adding' water vapour to dry air. If you take a certain blob of air and add to it a certain blob of water vapour, of course the combined blobs will be heavy than either of the original blobs. But if you take a blob of air of fixed volume, you can't add anything to it without pushing some of the air out.

[Mark 1]

The differences are pretty small though. At 15 degrees (standard day), the difference in density between completely dry and saturated air is about equal to 100' in altitude. The effect roughly doubles/halves for every 10 degrees increase/decrease in temperature.

Our UK weather is, generally speaking, continuously moist, with about 70-80% relative humidity typically.

[GOLF_BRAVO_ZULU]

tiggermoth

What Cobalt wrote was pretty close to your answer. You need to bear in mind (and if you don’t like physics, stop now) Avogadro's Hypothesis. The basis of that is that 1 molecule of any gas occupies the same volume, at the same temperature and pressure. Cobalt was adding atomic masses (N = 14, O = 16 and H = 1) to arrive at a molecular mass. Also bear in mind that gaseous Nitrogen is 2 Nitrogen atoms bonded to have an equivalent molecular mass of 28. With that in mind, consider that a water molecule has a molecular mass of 18 (1+1+16), so a given volume of that is 9/14 the density of Nitrogen, at the same temperature and pressure. If you want to complicate it, the molecular mass of oxygen is 32.

[Gipsy Queen]

For some reason these things are not noticeable on modern cars but when we drove around in 12hp side valve things, there was a very evident improvement in performance in a cool and damp evening compared with a dry mid-day.

The same could also be detected with small aero engines and I recall an occasion years ago when I had to wait for just such conditions in which to get airborne in a Miles Gemini. This had the Blackburn Cirrus Minor and performance was less than electrifying at the best of times so the increased volumetric efficiency - however you define it - was a boon.




Oh dear - I fear I have just demonstrated some rather bad airmanship! But there were extenuating circumstances . . . :rolleyes:

[IO540]

For a proper understanding, you just can't beat the good old steam tables :) :)

[tiggermoth]

But my steam tables are for saturated or superheated steam. :8 Ideal if I'm flying through a cloud at 32'C at almost full vacuum. (Sorry, I jest).

Having said that, if I'd paid more attention to things like steam tables and the like, then I wouldn't be asking such daft questions now. :O

[JP1]

Tiggermonth, I guess from your comment you have it now! As said above it is basically the molecular weight of the particular gaseous particles that is important and density = Molecular mass/volume. If you have X particles in a given volume, then that solid/liquid/gas has a certain density. Replace some of those particles with lighter ones (but still keeping the same number x), then the mass has reduced and hence the density.

Where your comparison with sponge has gone wrong, is that if you consider the dry sponge to be a container, its just containing air molecules (ignore the sponge material). When you add water you are replacing an air molecule not with a single water molecule (of less mass) but a collection of water molecules that are so tightly coupled, that they are in the liquid state and in effect the volume occupied by the single air molecule has been replaced by many water molecules, but still only using the volume of the original air molecule.

As an aside if you are slightly confused about the H20 molecules being in a liquid or gaseous state, its all down to the energy that the molecules have. Molecules are bounded by weak forces (convalence bonds) when in a liquid state. If you give the gas energy (by heating it) the molecules are able to break these weak bonds and the liquid changes to the gaseous state. Hence why clouds from at a certain height. The height at which the temperature drops to a point that the water molecules do not have enough energy to maintain broken covalence bonds, hence the water molecules return to the liquid state as water droplets forming the cloud.

[slim_slag]

Molecules are bounded by weak forces (convalence bonds) when in a liquid state.Water is amazing stuff, it's the huge importance of hydrogen bonds between molecules which give it very special properties. If it didn't have such properties there would probaby be no life as we know it.

[BEagle]

If saturated air was denser, clouds would always be at sea level!

Therefore it must be less dense than unsaturated air.

[slim_slag]

by that logic would't all the clouds be at the edge of space? Suspect temperature is a factor too.

Re water density. If water wasn't "special" all the ice would sink to the bottom of the seas and fill them up and we would be totally buggered. Water is densest at 4C, not many molecules behave that way, and very important that is too.

[JP1]

Molecules are bounded by weak forces (convalence bonds) when in a liquid state.

Too much wine and watching X-factor!!

I was refering to the general bonds between adjacent molecules and meant Van der Waals bonds if that term is still used.

I guess Tigermoth better not read any more

[tiggermoth]

I guess Tigermoth better not read any more

No, I'm really finding this all very interesting.

"Isn't water brilliant!"

I understand now why clouds don't go lower - because lower down the temperature is a bit warmer and so the air has more capacity for holding water, but then if the cloud tries to go higher then ..erm.. it hits the dew point and water droplets go sky diving.

I blame Avagadro. As for Pratt, in his Met book, he just says "Moist Air is less dense that dry air", and then does a runner!