Talk:Water vapor/Archive 4

Archive old talk - last comment of substance dated Nov 2003. Vsmith 15:14, 13 Feb 2005 (UTC)

Check...

Please check sources, links, etc. before randomly deleting or introducing non-existant links, however it was ironic that the bad scientific topic link turned out to be a bad link...humorous...I like that.

Topics on capacity of air to hold water vapor--see; partial pressures, vapor pressure, humidity, absolute humidity, specific humidity, relative humidity, Standard temperature and pressure...or even the water vapor article itself...

--Hard Raspy Sci 05:16, 14 Feb 2005 (UTC)

That was funny. But, the link (Bad Clouds) is a valid link - maybe you hit 'em during maintenance? The site has been a good resource for years. If those other articles contain the misconception then they need fixing also - will check 'em out later. A quote from the site:

The idea that it is the air which determines the amount of water vapor which can be present through some sort of holding capacity is an eighteenth century idea which was shown to be false both empirically and theoretically about two hundred years ago! The fact that it is still taught in our schools and defended by teachers and (gulp) professors, is a testimony to the mindless persistence of myth.

Vsmith 12:36, 14 Feb 2005 (UTC)

After, some thought, I disagree with the above quote. And I take back some of my following argument concerning air as a container. One simple fact is that air is a fluid, and the second, more complex, fact is that water vapor does not affect the heat capacity of air.

Fluids can contain other solids or fluids. Granted, getting them out is different than simply pouring out a glass of milk on a countertop. You have to visualize a fluid as a 3 dimensional container where each wall is either a wall or an opening.

Water vapor has its own heat capacity, irregardless of where it is. When it is added to a volume it does not change the heat capacities of the parts of the original air volume(ie Nitrogen, Oxygen, Carbon Dioxide, etc). The total heat capacity only changes as the sum of all the individual heat capacities change. Meaning that through thermodynamic manipulation, water vapor can be placed in, held in, or removed from the atmosphere. I think thats a definition of a container. Which means that a parcel of air is a container, as well as the Earth's atmosphere.

Also, web sites that focus on debunking, often times need debunking...because they only use a maximum of High School level science in an attempt to debunk a scientifically complex idea, which unfortunately may be already correct.

Hard Raspy Sci 08:38, 5 November 2005 (UTC)

Atmosphere vs. Air

The removed text:

"One limitation is given by the capacity of the atmosphere to hold water vapor"

is technically correct, and even for all cases. I read through the links and found the reasons they give as factually accurate. However, the minor differences are between the text here atmosphere and the misconceptions that concern air.

1st, atmosphere and air are not synonymous in all cases. Granted a general Earth-born mis-usage for atmosphere means air. Technically, an atmosphere can be any compilation of gases. But that's just a minor definition argument.

In all cases, atmosphere is not air, but all instances of air can be contained in an atmosphere. Where the atmosphere can be closed or open(planetary), and located on another planet other than Earth.

• But I admit error in my first hasty rebuttal: "capacity of air to hold water vapor"

It should have read "capacity of an atmosphere to hold water vapor"

It is true that ideal gases do not contain other ideal gases or have a capacity to hold other ideal gases. As the links you provided do express that, which is true especially under the current understanding of thermodynamics. The misnomer or misconception probably came from the misunderstanding of gaseous diffusion rates through other gases: that are contained in an atmosphere. While they are contained in an atmosphere, the gases neither disolve nor are disolved by other gases. Which is what we both agree to, I'm certain, as air is not a container.

An atmosphere is a region where any number of gases may be present. The capacity of that atmosphere to hold any gas is given by its boundaries and its thermodynamically available states. And thermodynamically available states are bounded by pressure, temperature, and volume, and mostly not on the presence of different gases.

To get the right idea of physical boundaries, boundaries would include solid and liquid surfaces, and a gravity well (ie. Earth's gravity). Also phenomena called temperature inversions between gases create temporary boundaries.

I would compromise in changing the text to read:

"One limitation is given by the capacity of an atmosphere to hold water vapor"

However, as I think about it, the example by Babin is a misnomer. It discussed a fresh water source vs. a salt water source. Its conclusion was in fact inaccurate. The example has absolutely nothing to do with the gaseous volume above the liquids. The only source for atmospheric water vapor from liquids below boiling is from surface available water molecules. Which really means, under same conditions the salt water is under net evaporation more often.

--Hard Raspy Sci 17:34, 14 Feb 2005 (UTC)

(William M. Connolley 17:54, 14 Feb 2005 (UTC)) I disagree with your suggested change. The atmosphere does not "hold" water vapour, you accept that, so there is no point inserting text that sugegsts this mistake. Perhaps the Babin page should be linked to directly in the article text to point this out. Also... the current texts says that atmos P/T determine saturation. Surely this is wrong? P is irrelevant.

On atmosphere: correct, which is why I didn't re-revert immediately because I had a problem still with the wording. What I was brooding over was the question--"what is the container". I asserted that atmosphere is the container, and air is the contents. My assumption would be air has no capacity, but atmosphere has capacity to hold.

The question is Is atmosphere the container or the contents?

On P is irrelevant: P as the pressure of the mixture is irrelevant, the vapor pressure and partial pressure is relevant. Vapor pressure is a function of T...partial pressure for water vapor is vapor pressure...to get RH, saturation vapor pressure is needed, which is also T dependant...and saturation vapor pressure is also a partial pressure...I guess it should be explored if the above Wiki links are correct with this notation.

• • definition from the American Geophysical Union

Water Vapor Terminology

Water vapor is water in the gaseous phase. Meteorologists have defined several different terms to express the amount of water vapor in air. Some refer to the actual amount, or concentration, of water vapor in the air, and some relate the actual amount to the amount that would saturate the air. Air is said to be saturated when it contains the maximum possible amount of water vapor without bringing on condensation. At that point, the rate at which water molecules enter the air by evaporation exactly balances the rate at which they leave by condensation.

So this must mean that gaseous diffusion rates are so badly misunderstood, they are implying that air is a container by effect of Brownian motion and collisions with other gases?? or something along those lines...

Oh crap on it, I'm going to bed--Hard Raspy Sci 03:42, 15 Feb 2005 (UTC)

The terminology is confusing. The same AGU site [1] also states:

The partial pressure of a given sample of moist air that is attributable to the water vapor is called the vapor pressure. The vapor pressure necessary to saturate the air is the saturation vapor pressure. Its value depends only on the temperature of the air.

If we would substitute equilibrium vapor pressure for the confusing saturation vapor pressure and avoid the somewhat misleading saturation concept it would be more clear. I have tried to do that in the editing I just did - hopefully for clarity. As for the contains highlighted above: yes, the water vapor is within the atmopshere and thus contained in it, but, it isn't held by anything - it's just there as part of the overall gaseous mixture. Vsmith 04:29, 15 Feb 2005 (UTC)

Right, which has to do with WV's heat exchange properties. So really, saturation came from chemistry when dealing with precipitates. So I imagine the analogy went as droplets are formed they precipitate from a saturated "region" . Then saturation itself is the misnomer, because saturation region would actually be a body of water, which has a surface tension.

Ok, I'm cool with it now...

--Hard Raspy Sci 15:36, 15 Feb 2005 (UTC)

Equilibrium vs. Saturation

I still have a problem with replacing saturation with equilibrium when talking about vapor pressure or partial pressure of water vapor. Equilibrium can be met at any point (saddle point or min / max). Saturation will only be met at one point (temperature) and in association with its partial pressure.

It should be clear that air over liquid water is in saturation, but it is not necessarily in equilibrium. For this reason, I think its ok to use saturation, properly, because it gives a sense of dynamics to the problem. For instance, an RH of 75% may be measured, and the air may be in equilibrium (condensation = evaporation).

But here is the kicker-- Lets say above the surface of a lake the air temp is 25°C and RH is 100% in the region right above the surface. [Forget wind] Right near the surface where RH is at 100%, equilibrium is not present.

Why? The water(liquid) is trying to seek heat equilibrium, because it is absorbing solar heat. So a net evaporation is occurring to cool the surface of the lake. The short version--sun heat in, water vapor heat out. The reason the region just above the lake is not in equilibrium has to do with gaseous diffusion and the rate at which the lake is being solar heated. The two are independent of one another. And saturation (near the surface) does not stop the water from evaporating. The improtant thing to remember is that evaporation and condensation will go into equilibrium when the temperature of the air and water agree.

Hard Raspy Sci 03:50, 2 Mar 2005 (UTC)

formula validity range

I wonder in which temperature range the formula for the saturation vapor pressure is valid? Is it valid below zero degrees Celsius?

(William M. Connolley 13:38, 6 Jun 2005 (UTC)) That formula is a bit of a weird version. The article really ought to make it clear that the basic dependence is exponential, with minor tweaks to fit to obs. http://www.natmus.dk/cons/tp/atmcalc/atmoclc1.htm has a rather easier-to-understand formula which would do. There are lots of different formula you can use, none of them are exact. They apply below zero oC.

Steam

I think i have nothing to say. --Saperaud 14:30, 30 July 2005 (UTC)

well i think that there is about 1% water vapour in the air around coasts and near the coast