måndag 31 januari 2011

Is the atmosphere an ideal gas, a blackbody or maybe something else?

Mixing or confusing physical concepts can be devastating. In previous posts we have seen examples of  such confusion which appear to be almost semantical. In my view, many misunderstandings could be avoided if

2. It was made clear whether the incoming sunlight should be treated as a heat flow or as an energy source.

Moreover, there seems to be a mixing of physical models. On one occasion (radiation) the atmospheric layers are treated as black-bodies, on another occasion (convective overturning, adiabat etc.) they are suddenly transformed into an almost ideal gas. If you follow the links you will discover that their thermodynamic properties, like pressure and heat capacity, differ in a qualitative way. 

However, is there any other way forward? In the end we must have a model that takes into account both radiation and kinetic energy of air molecules. Could such a model be that of a "boson gas"? The idea would be to treat all thermal excitations in the atmosphere, including photons, phonons, molecular momentum and so on as one kind of particle: boson. What would it be like?  

Since the number of bosons is not limited we would have to treat it as a grand canonical ensemble. As a matter of fact, papers have been written on this subject: 

Some excerpts:

Can anyone make sense of this?

1 kommentar:

  1. Hi Anders,

    Your post makes the source of your confusion quite clear, though perhaps not intentionally.

    You seem to assume that, because gases in the atmosphere radiate heat, they must be treated as 'black body' sources. But that does not follow at all.

    Atmospheric gases would not be treated as ideal 'black body' sources, since they are far from having the perfect absorption characteristics of a 'black body'. Nonetheless, they can radiate heat. If you take these two properties, call them A - 'is an idealised black-body source' and B - 'is a body capable of radiating heat', then any given body with property A must also have property B, but any given body with property B need not have property A. Simple.

    Also, you seem to confuse the notion of a molecular gas that radiates heat with the notion of a 'photon gas', but this is something quite distinct. I notice you link the words 'black-bodies' in your post to wikipedia's article on 'photon gas'. To help your confusion, I recommend you go one step further, and actually read it!