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Life Sciences => The Environment => Topic started by: Petrochemicals on 07/11/2019 00:52:48

Title: What is the PRECISE mechanism of atmospheric air heating ?
Post by: Petrochemicals on 07/11/2019 00:52:48
This is more complicated than it appears. I am wondering how air remains so cool.

How does the air in the environment become heated, how precisely does energy get into the air?

Radiation -Solar radiation, it hits the air to some degree, how much hits and is retained in temperature increace ? What limits temperature increace.

Conduction - how does conduction work. The adjacent air and  the surface of the planet heat the air, but how easily? How willing is the air to be heated by conduction. Lower pressure wind air I imagine would be at the same temperature so would not be inclined to draw energy from the land

Convection, how does convection heat the air, ( I can only think of water vapour, but any other ideas would be helpful, convection is by fluids such as air by definition) and how easily is this acieved.


Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: alancalverd on 07/11/2019 10:11:51
At the lowest levels, principally by convection. Sunshine and the earth's inherent volcanism and radioactivity determine the heat input to the surface, which transfers energy by conduction and radiation to air molecules near the surface. Where there is a boundary between an absorbent  hot (brown field, sand...) and a reflecting cool (green field, sea...) surface, cool air slides under the hot air to generate a thermal vortex that rises and mixes to produce a warm atmosphere. 

Adding water (up to 10% of air is water) complicates the issue. It requires more heat to raise the temperature of wet air by a given amount, i.e. to start the convective process, and as the convection bubble rises and cools by expansion, the water changes state to liquid then solid, releasing its energy in the upper troposphere and increasing the speed of the convective vortex.

There is some degree of direct radiative heating, principally of the water molecules which in all states have a very broad  absorption spectrum. This can sometimes be seen as winter low-level morning fog "burning off" from the top when the ground is very cold, whereas in summer it usually "lifts" from the bottom.

The huge swing of desert temperatures from overnight freezing to 40 deg C or more during the day, compared with, say, UK temperatures where the variation may be less than 2 degrees,  is clearly due to the absence of water in the desert atmosphere. 
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: Bored chemist on 07/11/2019 18:13:06
What limits temperature increace.
The other side of the Earth is radiating heat into space.
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: Petrochemicals on 10/11/2019 01:08:24
Thank you alan

The long wave radiation that is returned from the surface to space I conclude then from the desert principle,will largley pass through the nitrogen and oxygen unhindered? Or is it absorbed and re-emitted?

The surface conduction, how easily is this heat tranfer achieved ? Water seems very easily evapourated for energy transfer (sweat, wet clothes) but this would not raise the temperature. How rasily does the oxygen nitrogen mix draw heat from the ground?

The covection does not seem to exist as air is fluid itself, just hot risers transferring hot masses , would you agree ?

Interesting point about water being very hard to raise in temperature, 1 degree warmer means 10 percent more water can be held, this is a significant energy increace for little temperature increace, no suprise to the increace in atmospheric turbulance.
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: alancalverd on 10/11/2019 08:18:55
All molecules have an infrared absorption spectrum, but rigid dimers (O2, N2) and linear molecules (CO2) have far fewer distinct modes than a "bent" molecule (H2O) that can also form temporary polymers, not to mention liquid and solid phases in the atmosphere. 

Evaporation transfers energy from the solid surface to the atmosphere, so reduces the temperature of the surface and increases that of the surrounding air.

Dry convection happens but it is less obvious and generally less energetic than wet convection.
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: Bored chemist on 10/11/2019 10:45:20
All molecules have an infrared absorption spectrum, but rigid dimers (O2, N2) and linear molecules (CO2) have far fewer distinct modes than a "bent" molecule (H2O)
No.
Not all molecules absorb IR.
In particular, N2 and O2 don't (there is a technical exception for molecules where the two atoms are  different isotopes- I don't know if it has ever been observed.)

It's fairly easy to calculate the number of vibrational mode for a molecule. Count the atoms.
Each of the n atoms can move along one of 3 axes.
So, in total there are 3n  ways in which the molecule can move.
But 3 of those correspond to translations,.
Then you need to look at the motions which are rotations.
Ordinarily you can say that there are three rotations so the number of vibrational modes drops by 3
So that gives you 3n-6 possible vibrational modes.

There's an exception in the case of linear molecules- rotation about the linear axis isn't meaningful because there's no change when you rotate it that way. That rules out one of the 3 rotational axes.
So, in the case of linear molecules there  are 3n-5 vibrational modes.

So let's compare water and CO2.
Water has 3 atoms so n=3  and 3n-6 = 9 -3 = 3 fundamental modes.
CO2 also has 3 atoms in the molecule and again n=3,
but it is linear molecule so the equation concerned is 3n-5
3n-5 =9-5 = 4 vibrational modes.

So what you have said is that 4 is far fewer than 3.


https://en.wikipedia.org/wiki/Molecular_vibration

Evaporation transfers energy from the solid surface to the atmosphere, so reduces the temperature of the surface and increases that of the surrounding air.
No
The latent heat of evaporation heat comes from somewhere and it's taken from the surroundings.
Both the air and surface  of the solid (or more commonly liquid- solids don't typically evaporate much) are cooled.
Dry convection happens but it is less obvious and generally less energetic than wet convection.
You need to define what those mean.
Once you get a phase separation- the production of rain or ice- and that dense phase falls youno longer have convection- you have advection.
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: alancalverd on 10/11/2019 18:10:40
You had better inform the people who publish the absorption spectra of gases that they are wrong, before they confuse the next generation of students.

Best not to sweat, because the evaporation won't cool you.

Fog is liquid water that doesn't fall. Like cloud, only lower. And ice fog is particularly interesting.
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: Bored chemist on 10/11/2019 18:23:39
You had better inform the people who publish the absorption spectra of gases that they are wrong, before they confuse the next generation of students.
It's OK they will read wiki and find out you are wrong.

Best not to sweat, because the evaporation won't cool you.
It cools you down just fine.
Typically you don't evaporate much, but the water does.
Also nobody said it didn't cool you down.
But what is important is that it does not heat the air. It cools that too.
That's why people use "swamp cooler" air coolers.
https://en.wikipedia.org/wiki/Evaporative_cooler

You are quite simply wrong to say "Evaporation transfers energy from the solid surface to the atmosphere, so reduces the temperature of the surface and increases that of the surrounding air." and trying to double down on it doesn't make you look smarter.

Fog is liquid water that doesn't fall. Like cloud, only lower. And ice fog is particularly interesting.
Yes, we know that.
And...?
Title: Re: What is the PRECISE mechanism of atmospheric air heating ?
Post by: Bored chemist on 10/11/2019 18:25:01
the people who publish the absorption spectra of gases
OK, let's start with the IR spectrum of N2 or O2...
See if you can find me a copy.

Just to clarify; two is not four.