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Author Topic: Why is rising atmospheric pressure associated with drier air?  (Read 7559 times)

Jeff Thompson

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Jeff Thompson asked the Naked Scientists:

According to my barometer, as the atmospheric air pressure increases from 29 to 31 psi, the weather goes from 'stormy' to 'rain' to 'fair' to 'very dry', indicating that wet air weigh less than dry air. This seems counter-intuitive.

Please explain.

What do you think?


 

paul.fr

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Why is rising atmospheric pressure associated with drier air?
« Reply #1 on: 18/12/2008 13:12:45 »
wet,humid, air does weigh less than dry air. I think the confusion arises because we are used to thinking of water as something you hold in your hand and you get an actual feel for the weight, yet you think of air as a gas, something you associate with having no weight.

these links may help:

http://www.wrh.noaa.gov/fgz/science/cloud.php?wfo=fgz
http://ga.water.usgs.gov/edu/watercycleatmosphere.html
 

lyner

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Why is rising atmospheric pressure associated with drier air?
« Reply #2 on: 18/12/2008 15:56:17 »
I'm not too impressed by the statement
"Therefore, a cloud should "weigh" less than the air it is displacing. "
in the first link. If it weighed less than the air it displaces, it would float upwards! It must weigh the same (have the same density) if it stays at the same height. Their reasoning is too simplified.

At the same pressure and temperature, all gases (and mixtures of ) have the same number of molecules per unit volume. Water molecules, having two Hydrogen atoms (H2O) are less massive than the diatomic molecules of Nitrogen (N2) and Oxygen (O2). So water vapor and mixtures containing water vapor are less dense than air.  This is not really 'damp air' but it may be nearly saturated. The dampness will only occur when there is condensation due to a drop in temperature
Droplets of water, which have condensed out of the vapor, actually add to the total density of a cloud. i.e. Nitrogen, Oxygen and Water Vapor molecules PLUS some water drops are involved. The volume of the mass of air will have decreased.
To get the air to rise from where it started,  you can push in a supply of cooler, more dense, air in underneath it, in the form of a 'front' or you can warm it up - supplying energy from a hot patch of ground, for instance. This energy will cause it to expand and be pushed up by the surrounding cooler air.  This is how Cumulus clouds often form. The actual height to which the cloud will reach depends, ultimately, on the amount  of thermal energy which has been supplied - which is transferred to gravitational potential energy used up in lifting it.

There is another bit of folklore which is misleading about dryness of air. It is often said that "warm air can hold more water than cool air". Whilst this may appear to be the reason for  what we observe,  the real explanation is that the vapor pressure of water increases with temperature so more water will appear in the form of vapor when it is warm than when it is cold. The same pressure of water vapor could be measured if you did the experiment in an evacuated bell jar as the 'partial pressure of water in  atmospheric air at the same temperature. It's the water that's doing it - not the 'air'.
« Last Edit: 18/12/2008 16:01:10 by sophiecentaur »
 

paul.fr

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Why is rising atmospheric pressure associated with drier air?
« Reply #3 on: 18/12/2008 18:45:15 »
I'm not too impressed by the statement
"Therefore, a cloud should "weigh" less than the air it is displacing. "
in the first link. If it weighed less than the air it displaces, it would float upwards! It must weigh the same (have the same density) if it stays at the same height. Their reasoning is too simplified.

Yes, but the given explination was simplified because of the audience it was / is aimed at. If you are searching for some questions then it can be assumed that your level of knowledge is basic, you then get a basic answer. Should this not satisfy your curiosity then you can always ask again, explaining what you already know, or by using the language that would tell the reader you have some level of knowledge.

Quote
There is another bit of folklore which is misleading about dryness of air. It is often said that "warm air can hold more water than cool air". Whilst this may appear to be the reason for  what we observe,  the real explanation is that the vapor pressure of water increases with temperature so more water will appear in the form of vapor when it is warm than when it is cold. The same pressure of water vapor could be measured if you did the experiment in an evacuated bell jar as the 'partial pressure of water in  atmospheric air at the same temperature. It's the water that's doing it - not the 'air'.

Not sure if this is folklore, or just easy teaching, or again -teaching to a specific level of education. Again, as above, it is a simplified answer that is easy for people with little or no knowledge. Unfortunately, these kind of snippets are easily digested and remembered, even if they are not truely accurate.

(probibly) the best reply you will find is this

http://www.ems.psu.edu/~fraser/Bad/BadClouds.html



Bad Meteorology:
The reason clouds form when air cools is because
 cold air cannot hold as much water vapor as warm air.

When moist air cools, a cloud can form. This much is true. The process is responsible for the cumulus cloud over Vancouver and the cap cloud over Rainier, shown to the right. Ascending air always cools. The cumulus cloud formed when air over the sun-warmed ground became buoyant and rose; the cap cloud, when the wind (coming from the right) blew against the sloping side of the mountain and was forced up.

But did the clouds form because the colder air had a lower holding capacity for water vapor than the warm air? If you believe a legion of teachers (from grade school to university), TV weather broadcasters, and endless textbook writers, this is the reason. They speak of the air being saturated and one even published an illustration of the air being wrung out like a sponge as the temperature dropped (sigh...). Unfortunately, it is not true. Sure, a cloud may form as the temperature drops, but not because some mystical holding capacity of the air has decreased.

To claim that a temperature-dependent holding capacity of the air caused the cloud to form in cold air is to get (approximately) the right answer for the wrong reason. It is like trying to reduce the fraction, 19/95, by imagining that you can cancel the 9s. The right answer ensues, but for the wrong reason. And, if the process was wrong, it is unlikely to work the next time you try it in a slightly different situation.

The air (mainly nitrogen and oxygen) no more has a holding capacity for water vapor, than, say, water vapor has for nitrogen. The atmosphere is a mixture of gases. While saturation (which involves bonds between different molecules) is a real phenomenon in liquids it does not describe the interaction of atmospheric constituents.



So, what is going on?

Water molecules are constantly coursing back and forth between phases (another word for the three states: vapor, liquid, and solid). If more molecules are leaving a liquid surface than arriving, there is a net evaporation; if more arrive than leave, a net condensation. It is these relative flows of molecules which determine whether a cloud forms or evaporates, not some imaginary holding capacity that nitrogen or oxygen have for water vapor.

The rate at which vapor molecules arrive at a surface of liquid (cloud drop) or solid (ice crystal) depends upon the vapor pressure.

The rate at which vapor molecules leave the surface depends upon the characteristics of the surface. The number escaping varies with:

           1. the phases involved --- molecules can escape from liquid more readily than from the solid (ice);
           2. the shape of the boundary --- molecules escape more readily from highly curved (small) drops or ice crystals (convex);
           3. the purity of the boundary --- foreign substances dissolved in the liquid or ice diminish the number of water molecules which can escape;
           4. the temperature of the boundary --- at higher temperatures the molecules have more energy and can more readily escape.


And therein lies the origin of the myth. The temperature of a cloud droplet or ice crystal will be (nearly) the same as that of the air, so people imagine that somehow the air was to blame. But, if the (other gases of the) air were removed, leaving everything else the same, condensation and evaporation would proceed as before (the air was irrelevant to the behavior). To assign the behavior of water to an invented holding capacity of the air is like assigning your life's fortunes to an invented influence of the constellations (and as we all know, nobody does that anymore).

So, what do you tell your students?
What appears to be cloud-free air (virtually) always contains sub microscopic drops, but as evaporation exceeds condensation, the drops do not survive long after an initial chance clumping of molecules. As air is cooled, the evaporation rate decreases more rapidly than does the condensation rate with the result that there comes a temperature (the dew point temperature) where the evaporation is less than the condensation and a droplet can grow into a cloud drop.

Evaporation increases with temperature, not because the holding capacity of the air changes, but because the more energetic molecules can evaporate more readily (with, of course, the caveat that evaporation is also influenced by things other than temperature, as described above).

If that explanation is not simple enough for your students, just present the facts: when the temperature drops below the dew-point temperature, there is a net condensation and a cloud forms.

But don't ever teach nonsense by claiming that the air has a temperature-dependent holding capacity for water vapor.

A little history

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. A discussion of some of the history of this bankrupt idea is offered by  Steven M. Babin .

       

 

Offline Bored chemist

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Why is rising atmospheric pressure associated with drier air?
« Reply #4 on: 18/12/2008 18:51:03 »
Jeff Thompson asked the Naked Scientists:

According to my barometer, as the atmospheric air pressure increases from 29 to 31 psi,....

Please explain.

What do you think?
I think that he's living 60 feet under water or he means inches of mercury rather than PSI.
 

lyner

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Why is rising atmospheric pressure associated with drier air?
« Reply #5 on: 20/12/2008 12:14:28 »
Paul
Very succinct little article, explaining things as they 'actually are'.


As for you comments on answers being aimed at an audience.
There are plenty of much better and understandable answers available than the ones that the conventional teaching system give. There is no reason to ignore Newtons laws and nor is their any point in describing air as a 'sponge' of some kind.
You might as well justify saying "things naturally Slow down" because that is a common experience and wouldn't provide any cognitive conflict.

UK Science Education claims to be telling kids "How Science Works", these days. Why not do just that? Possibly because it might involve some self education for a host of people whose job it is to know and to teach it.
 

paul.fr

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Why is rising atmospheric pressure associated with drier air?
« Reply #6 on: 06/01/2009 15:29:02 »
Quote
Paul
Very succinct little article, explaining things as they 'actually are'.

Yes it is, and unlike others (myself included), the author explains in such a way that makes it easy to understand.
I have referenced and recommended the site for ...a long time.


Quote
...UK Science Education claims to be telling kids "How Science Works", these days. Why not do just that? Possibly because it might involve some self education for a host of people whose job it is to know and to teach it.

May be I am misunderstanding where you are coming from, but aren't subjects are taught to the age, and level of education of the student.

Do UK schools still teach that "water freeze at 0 degrees c". This, as you know, is not the full answer, and not wholly correct. Yet it is (was) taught and stated as an absolute truth.
 

lyner

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Why is rising atmospheric pressure associated with drier air?
« Reply #7 on: 06/01/2009 22:05:36 »
Paul
What is on the curriculum is thought to be appropriate for age and level of ability - by educationists and politicians.
However, what is taught depends largely on the knowledge and ability of the teachers and the grasp of Science which the course inventors happen to have - it can't be better than that, can it?  Have you read a modern Key Stage 3 text book recently and looked at it 'as if' by a child?

Quite frankly, since GCSE came along and since three Sciences were required to be taught to all students up to 16, it has gone downhill. Choices are not possible and there is not enough timetable time for three separate Sciences to be taught as well as the the other necessary subjects.
Only a minority of students are taught anything harder than 'double award' science, which means that they get, what was, three (already simplified) subjects delivered in the timetable allotted to two. I know of no School which delivers three separate Science GCSEs without taking some 'special' measure - like extra lessons after School or a 'cramming' session for the bright ones.

On top of that, many students may not be taught by a teacher with a specialty in one of the three Sciences EVER IN THEIR SCHOOL LIFE. How can that be excusable?
So we (could) have a Biology Teacher (it could be Chem or Phys) who says they "hate" teaching Physics and "never did understand it" delivering two years of GCSE Physics to a student who then needs to choose which A level subject they want to study. Sods law could have it that, in the previous three years, that same student had: 1. A middle-school qualified 'Science' teacher without an A level in any Science, even:  2. A biology teacher:  3. A Chemistry teacher.

"Heat rises", "air is like a sponge", "Electicity and water don't mix" etc.; they all get told to kids.
Is it fair?

Oh yes, and we also have to fit all the social aspect of Science bollocks plus "the Scientific Method"  at the same time. There is, basically, no room for good old Newton and his mates.
Ranting over now but I have thought this through over and over again.

« Last Edit: 07/01/2009 13:36:18 by sophiecentaur »
 

paul.fr

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Why is rising atmospheric pressure associated with drier air?
« Reply #8 on: 07/01/2009 21:46:19 »
Paul
What is on the curriculum is thought to be appropriate for age and level of ability - by educationists and politicians.
However, what is taught depends largely on the knowledge and ability of the teachers and the grasp of Science which the course inventors happen to have - it can't be better than that, can it?  Have you read a modern Key Stage 3 text book recently and looked at it 'as if' by a child?....

Ah, I see where you are coming from. My apologies for being slow on the uptake.
 

lyner

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Why is rising atmospheric pressure associated with drier air?
« Reply #9 on: 07/01/2009 23:49:17 »
Not at all; I just like to moan about it! ;-)
 

paul.fr

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Why is rising atmospheric pressure associated with drier air?
« Reply #10 on: 08/01/2009 17:28:17 »

"Heat rises", "air is like a sponge", "Electicity and water don't mix" etc.; they all get told to kids.
Is it fair?


Ah, I am sure you will enjoy this headline from the Guardian:

'I was sure water and electricity don't mix, but I didn't dwell on it'
http://www.guardian.co.uk/uk/2007/jul/25/weather.world
 

lyner

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Why is rising atmospheric pressure associated with drier air?
« Reply #11 on: 08/01/2009 18:30:39 »
Heroes are allowed to use jargon!
 

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Why is rising atmospheric pressure associated with drier air?
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