How hot does Gaseous H2O need to be to emit IR Photons?

[Michael Thornton]

I've read that H2O has an absorption/emission band around to 10 micron range. What conditions are required for photons of this wavelength to be emitted by H20 gas? In particular, how hot would the gas have to be? What amount of pressure is required? Under everyday conditions (like steam possible to detect an appreciable amount of these photons?

Also, how would you determine this for any wavelength absorption/emission band?

Thanks for any and all responses!

 

[Bystander]

"Ten microns?" Room T.

 

[Michael Thornton]

"Ten microns?" Room T.

Ten micrometers

 

[russ_watters]

Bystander knows what a micrometer is: he was answering your (main) question. To answer the follow-up question, it is a function of black body radiation (equations on its wiki page), with the peak frequency corresponding to the temperature:

 

[Michael Thornton]

Ok, I guess I'll need to be more specific.

How much IR light would be radiated compared to a blackbody? I'm under the impression that water vapor is not a very good blackbody, so I'm trying to understand that idea a little better.

 

[russ_watters]

I don't have specific answers, but I'd look into this in the context of weather forcasting: ir satellite photos of water vapor is one of our better tools for it.

 

[klimatos]

Actually, water vapor is fairly transparent to the ten micron wavelength band. That is why that band forms one of the humid atmosphere's IR "windows". On the other hand, liquid water absorbs it quite nicely. This band is one of the major sources of terrestrial IR for clouds, and the reason cloudy nighttime skies are "warmer" than clear ones. They produce "back" radiation to the Earth's surface that is absent under clear skies. This radiation, in turn, comes in large part from the warming caused by the absorption of the ten micron band. This band is the peak wavelength in the surface IR emissivity.