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QUESTION: A layer of water vapour at 100 deg Celsius can emit more than 50% of that which a blackbody at the same temperature emits (Reference: Fundamentals of Thermal-Fluid Sciences by Cengel and Turner). What is the emissivity of water vapour at temperatures of, say, 500 deg C and 1000 deg C? I am asking you because I have tried science sites, I have searched the Internet and textbooks and I still do not have an answer. Could someone give me an idea about a metre thick layer of water vapour when it comes to emissivity. Even a general sort of answer, such as emissivity increases or decreases with increasing temperature of water vapour

ANSWER: Ummm...that doesn't make any sense.  An idea black body has a perfect emissivity of 1.  Objects don't emit more than that unless they're specifically stimulated in special ways (like the gas in a fluorescent light bulb) to do so.  The emissivity of water is very high, close to 1, but it's not 1.  I don't have access to your reference, but is it possible that you misinterpreted it?

*update: This reference may help you:

---------- FOLLOW-UP ----------

QUESTION: The gas molecules in the atmosphere (water vapour and so on) absorb and emit radiation. Although hydrogen, oxygen and nitrogen emit only small amounts at moderate temperatures, the carbon dioxide and water molecules absorb and emit substantial amounts, affecting climate. About what percentage of radiation is emitted by gases apart from carbon dioxide and water molecules?

I didn't want to just brush this off, but this is really pretty far outside my area of expertise.  I'm going to direct you here:  Also see here: You don't need a nuclear physicist for this, you need a climatologist.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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