# Gases that are nearly transparent to solar AND thermal radiation?

I've been curious about if there are any gases in the atmosphere that are nearly transparent to both solar and thermal radiation. CO2 for instance is nearly transparent to solar radiation but is partially opaque to thermal.

I've tried to find information on the internet, but the large amount of information about global warming is effectively hijacking my google searches.

Edit: When I say solar radiation, I am not just referring to that which is visible. Please consider at least infrared, visible, and UV in your answer.

Any help or insight will be greatly appreciated.

• He, Ne, Ar, Kr, Xe, $\mathrm{O}_2$, $\mathrm{N}_2$, ... Feb 12 '17 at 12:46
• Alternatively, en.wikipedia.org/wiki/Greenhouse_gas#Non-greenhouse_gases . Voting to close for insufficient research. Feb 12 '17 at 15:15
• I'm voting to close this question as off-topic because insufficient research was done before asking it. Feb 12 '17 at 15:16
• I'm pretty sure your comment isn't correct. Oxygen, for example, absorbes from a decent range of wavelengths from the ultraviolet radiation of the solar radiation. Solar radiation includes visible, inflated, UV, plus some less important. Feb 12 '17 at 17:19
• *Infrared, not inflated. Silly auto correct. Feb 12 '17 at 18:14

Google, Radiative (or radiation) absorbtion coefficients for gases (or enter a specific gas of interest). Most of this data relates to applications in combustion and refrigeration so that is the area to look at. To avoid "contamination" from all the global warming stuff try looking at older things like papers from the 1950's. I have some detailed refrigeration books from the 50's which I think had this data but are not at hand right now. Otherwise, see if you can get hold of some books like that from your library or a bookstore.

Many gases were studied in great detail with proper experimental measurements to verify data well before global warming was a thing. You can find graphs of lots of useful things which can help you with an understanding of how the various gases might work in the atmosphere. For example using the emissivity of CO2 to calculate radiative heat transfer in the atmosphere with the standard Stefan-Boltzmann equation when you know the concentration of CO2 is, say, 400 ppmV. Emissivity of CO2 can be surprisingly low at atmospheric concentrations, temperatures and pressures, less than 0.002 typically.

I highly doubt there's anything at all which would fulfil the criteria you're looking for due to the huge spectral range involved and an overlap between the two emission spectra.

You'd need something which doesn't absorb anything between 0.2 µm and 70 µm.

Alternatively you'd need something with two absorption bands of equal spread and magnitude sitting on equivalent intensity points of the two emission curves, but even then both emission curves are moving targets; the Earth's due to temperature changes and the Sun's due to various solar phenomena that effect the emission strength at specific wavelengths.

Not to the best of the my knowledge. Such a thing is called dark matter, which is almost undetectable. None of it exists in our atmosphere, at least that we know of.

• I think you are applying far too stringent a constraint on "nearly" :-) May 7 '17 at 18:23
• Dark matter is matter that interacts only gravitationally with other matter. That's a very far stretch from having weak opacities in the UV-Infrared. Aug 8 at 22:06
• @AtmosphericPrisonEscape The key word in that is only. It does not interact with electromagnetic radiation, therefore it would be transparent. Why do you think it is so difficult to detect? Aug 8 at 22:43

All radiation is thermal. I really can´t understand why anyone makes a difference between different parts of the spectrum. The "visual" part of radiation is entirely relative to human eye-sight. The definition of visual is exactly that, the radiation which the receptors in our eyes respond to. The concept "albedo" is also something entirely relative to human eye-sight, and it should be questioned if it is correct to use it for a planet in regards to temperature at all. For example, snow is like a perfect blackbody absorber outside the visual spectrum, but in the visual it is reflective. And solar radiation is 51% IR(thermal), so what use do we have of "albedo" when we calculate temperature? "Thermal" radiation is entirely based on the fact that human eyes doesn´t respond to those wavelengths, and the rest of the longwave radiation parts of the spectrum is called non-thermal mostly because humans can´t sense it at all.

"Solar radiation" cover the spectral parts where we say that the gases in question is opaque, although wavelengths are mostly in the higher frequency part. So the "transparent" parts apply to both solar and thermal, with your definitions.

The fact is, all radiation is thermal, intensity and wavelengths depends on the temperature of the emitter, only. Any part of the spectrum has an intensity according to the temperature of the emitter, spectral resolution and peak intensity of emission is also caused by the temperature of the emitter. The entire concept of radiation in the universe is an exclusive relationship where emission depends on temperature alone. If we know this, which we do, then all radiation is thermal. And nothing on earth, no matter what gases are involved, should interfere with that relationship, because it would be a violation of 100% consensus physics. Emission is dependent on temperature alone, absorption is due to temperature only. Spectral resolution is quantum physics, and we don´t use that for temperature. Now, if you want to discuss spectral character of something, that is fine. But if we are talking about temperature, of the atmosphere or earth surface, transparency should not be involved.

• Would you also consider gamma and radio waves thermal in the context of the greenhouse effect? May 5 '17 at 22:48
• Gamma and radio waves are never in the context of the greenhouse effect. The greenhouse effect is per definition in context of radiation. So, can you state your question a bit more clearly? May 6 '17 at 15:53