Monday 19 September 2016

Fun Online Polls: Atmospheric temperatures, politicians and sugar

The results to last week's Fun Online Poll were as follows:

Why is the earth's surface 30C warmer than it should be, and 60C warmer than the top of Mount Everest?

Greenhouse gases like H2O and CO2 - 20%
Gravity and Boyle's Law warm the lower and cool the upper atmosphere - 67%
Other, please specify - 14%

Which was of course the correct answer. I was told that more specifically I should have said Gay-Lussac's Pressure-Temperature Law:

"The pressure of a gas of fixed mass and fixed volume is directly proportional to the gas's absolute temperature."

(Though having re-read this, they seem to be saying the same thing.)

Atmospheric pressure at ground level is higher than at the highest altitudes, ergo it is warmer than it would be from sunshine alone, and at the top of the atmosphere, it is colder than the average temperature of the whole atmosphere.

Clearly heat can't come from nowhere so what the lower atmosphere gains (in terms of average number of molecules per unit volume with a correspondingly higher total kinetic energy of those molecules per unit volume, which can be measured as 'pressure' or 'temperature', same thing in this context) must be balanced out by lower numbers for density, temperature, pressure etc in the higher atmosphere .

There is an equilibrium, which depends on how thick the atmosphere is, the air at the bottom can only warm up so far before it rises and cools again; the air can only rise so far before the force of gravity pulling it down overrides its tendency to float up etc.

It's all well and good coming up with an explanation why the surface temperature is higher than it would be from sunshine alone (and yes, 'greenhouse' gases like H2O, CO2, CH4 contribute a couple of degrees but not a massive amount) but if that does not also explain why the upper reaches of the atmosphere* are colder than they would be etc, then that explanation is not coherent or plausible.

* Don't start with the thermosphere, that is high up but hot for very different reasons.
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Going back to a Fun Online Poll of a few weeks ago, 71% thought that Merkel would never realise it was wrong to let a million Muslim refugees into the country.

Interestingly, she sort of did today, although it wasn't really an apology as such, more that she regrets having lost votes over it and wishes she had prepared the ground better before she pressed ahead and did it anyway.
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On the topic of politicians, another politician-avoiding-sugar story popped up again today, I don't know why some people are so obsessed with other people's diets but hey, I try to avoid sugar where poss so he has my sympathy (although I don't make jam, don't really like the stuff).

From The Daily Mail:

* The Labour leader made the comment during a chat on Mumsnet
* Said he shunned biscuits because he was 'anti-sugar on health grounds'
* But mocked as he has a jam-making hobby, which usually involves sugar


Luckily, the internet never forgets. For contrast, there was another politician-avoids-sugar story a while back, which raised no eyebrows whatsoever.

So that's this week's Fun Online Poll (without following that last link first!)

"Which of these politicians avoids sugar?"

Vote here or use the widget in the side bar.

13 comments:

Dinero said...

Gas pressure can be caused by two things , either a higher number of molecules or higher kinetic energy of the individual molecules. Temperature can only be caused by one of those things, i.e. higher kinetic energy of the individual molecules.

Gay-Lussac's law says that temperature causes pressure it doesn't say pressure causes temperature.

The gas nearer the ground is warmer because it is nearer the ground which is the heat source.

James Higham said...

I should have said Gay-Lussac's Pressure-Temperature Law

A simple error any human could make. :)

Mark Wadsworth said...

Din, did you follow the second link?

"Gay-Lussac's law says that temperature causes pressure it doesn't say pressure causes temperature."

It's a simple fact that if you compress gas its temperature and pressure increase in tandem. You do not get this effect with liquids because they are not compressible.

"The gas nearer the ground is warmer because it is nearer the ground which is the heat source."

That explanation might satisfy a ten year old but is actually incorrect. The gas nearer the ground is permanently under higher pressure because of gravity ergo it gives a higher temperature reading.

Anyway, I'm bored now. Believe what you like, it's wrong.

JH, to my mind they say pretty much the same thing but a purist might disagree.

paulc156 said...

Basically MW just insists we don't have feedback mechanisms and assumes a perfect state of hydrostatic equilibrium. The latter is reality, the former is effectively restricting us to 18thC chemistry. Moral of the story: Assume a daft premise and you can demonstrate anything you set out to.

Bayard said...

"It's a simple fact that if you compress gas its temperature and pressure increase in tandem."

No they don't, nor does Gay-Lussac's law say that they do. G-L's law states that, at a given pressure, the volume of an enclosed gas is directly proportional to its temperature. If you reduce the volume of a gas (compress it), you will increase its pressure, but not necessarily its temperature (Boyle's Law).

As I have pointed out before, if you heat a gas by doing work on it, you will raise its temperature. Compressing a gas usually involves doing work on it, therefore compressing it makes it hotter. This has nothing to do with either Gay-Lussac's Law, Boyle's Law, Charles's Law or the Combined Gas Law. It is simply the same process as boiling a kettle, i.e. raising something's temperature by adding heat.

Graeme said...

Mark

try this https://scienceofdoom.com/2009/11/28/co2-an-insignificant-trace-gas-part-one/

Nothing wrong with what you are saying but it is oversimplified to base it on ideal gas theory and you need to take it down a level or 2 to understand the dynamics, given that temperature/pressure of the atmosphere, as measured, are in a state of continual change - just look at the data from any measuring station

Mark Wadsworth said...

PC, you have no idea what I "assume". From a cursory reading of your comments, I get the impression that your greenie assumptions are off the scale of any sort of reality. They are from la la land. I shall cheerfully ignore them if you don't mind. Oh you do mind? Ah well. If you have noting useful to add, don;t add it.

B, you also did not read the links from the previous post? When Boyle was doing his experiments on what happens when you compress a gas, the caveat is always AFTER COMPRESSING HE ALLOWED THE GAS TO COOL DOWN AND THEN MEASURED PRESSURE. Do you carefully ignore that bit for the sae o winding me up?

"Compressing a gas usually involves doing work on it, therefore compressing it makes it hotter."

Again, you refuse to accept reality or even bother reading my earlier comment which dealt with this.

"It's a simple fact that if you compress gas its temperature and pressure increase in tandem. You do not get this effect with liquids because they are not compressible."

G, I read the link, it completely ignores what I am referring to and goes full-Greenie and suggests that without H2O and CO2 in the atmosphere, the surface temperature would be -15C.

Of course these gases increase temperature somewhat. So what? It is a fraction of what the weight of the atmosphere adds.

You (and probably PC, to the extent that he actually has any thoughts of his own) appear to think that I am presenting this as an alternative to Greenie theory, that by putting forward this explanation I am truing to say that CO2 and H2O have no effect.

I am not saying anything of the sort. I am saying the atmosphere itself makes up the bulk of the effect, H2) and CO2 just add a couple of C on top.

And of course I am simplifying, I do live on planet earth and know that there is such a thing as 'weather' and that temperatures are always going up and down and the air swirls around etc.

Mark Wadsworth said...

B, further, your theory about "compressing a gas heats it because of work done" theory does not explain why gas in a sealed container cools down if the container is enlarged (i.e. a piston pulled out). That is a similar amount of work to compressing the gas, but miraculously, this time, the "work done" cools the gas.

Mark Wadsworth said...

B, maybe you should remind yourself how fridges work. Gases heat up when compressed and cool down when they (are allowed to) expand.

Dinero said...
This comment has been removed by the author.
Dinero said...

There are two different things getting mixed up in this thread .

1. Gasses are heated by the act of compressing them, which is True

2. Higher pressure gases remain warmer for as long as the higher pressure remains high because pressure causes temperature, which is False.

For example thermometers measure temperature but they don't measure pressure, and full scuba diving tanks are not warm to the touch.

Bayard said...

"B, you also did not read the links from the previous post? When Boyle was doing his experiments on what happens when you compress a gas, the caveat is always AFTER COMPRESSING HE ALLOWED THE GAS TO COOL DOWN AND THEN MEASURED PRESSURE. Do you carefully ignore that bit for the sae o winding me up?"

Yes I read the links. The reason why Boyle let his gas cool down was that he was investigating the relationship between pressure and volume at a fixed temperature, so he had to remove the temperature variable by letting the gas cool down.

The bit you are carefully ignoring is the second law of thermodynamics, which is that the entropy (disorder) of energy always increases, i.e. if you could increase the temperature of a gas without doing work to it, i.e. without introducing energy, then you would have decreased its entropy, high temperatures having less entropy than low ones.

OK, my comment above wasn't strictly accurate: compressing a gas increases its energy density and hence its temperature, but energy has to be put in in order to do this. (Conversely, expanding a gas decreases its energy density and its temperature, but energy still has to be put in to do this, so the cooling effect is reduced). However, in both cases work has to be done. Things have to be moved by an outside force. This does not apply to the atmosphere as there is no outside force (see my comment to the next post on this subject).

Mark Wadsworth said...

B, gravity is a force, the motion in molecules is energy, gas molecules are whizzing around and bumping into each other etc.

If gravity does not cause higher pressure and temperature at the centre, then you will also have to write thousands of letters to thousands of websites and academic publishers telling them that their explanation of how stars form and what it's like inside a gas giant are wrong.

Best of luck with that and please let us know how you get on.

To recap: in a fridge/heat pump, one unit of energy is used to pump ten units of heat from one place to another. The energy coming out of the back is more than the energy being used, because that energy is coming from inside the fridge. A 100% energy efficient fridge would still pump heat out of the back.

The extra energy at the bottom of an atmosphere is like the heat coming out of the back of the fridge - the flip side of this is that at the top of Mount Everest it is very cold and colder than it would be without this effect (i.e. -15C not -45C).

To have a vague shot at being correct, an explanation must not only explain why it is hot at the bottom but also cold at the top (and actual temperatures must back it up). You have come up with plausible explanations for why it is hotter at the bottom but as these do no explain why it is colder at the top, they are incomplete and incorrect. Sorry, but that's how science works.