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There are topics on how water vapour absorbs radiation ("How does carbon dioxide or water vapour absorb thermal infra red radiation from the sun?"), but what I am looking for is can water vapour also emits radiation in the following proces?

When water vapourize for example from the ocean, it contains latent heat. When water vapour rises into the atmosphere it can absorb radiation.

Will / can water vapour also emit heat in the form of radiation before it condenses into the liquid form? I know it will transmit radiation in the phase change, but I am only looking in the phase before the phase change.

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2 Answers 2

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Yes, it will. In general it can emit radiation at any frequency it absorbs it at, and it will do so. However this emission is essentially random, because the orientation of the molecules is random, so even if molecules don't emit radiation isotropically (ie if they don't emit radiation equally in all directions relative to the orientation of the molecule) the overall pattern is random.

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  • $\begingroup$ The direction is random even if we neglect scattering. Even if the atoms radiated in some anisotropic pattern, their orientation being random smooths this pattern to be isotropic. $\endgroup$
    – Ruslan
    Commented Jun 29, 2019 at 5:53
  • $\begingroup$ @Ruslan: thanks, I've amended my answer. $\endgroup$
    – user107153
    Commented Jun 29, 2019 at 8:34
  • $\begingroup$ "Not equally in all directions" is anisotropically, not isotropically. $\endgroup$
    – Ruslan
    Commented Jun 29, 2019 at 8:36
  • $\begingroup$ @Ruslan Doh, sorry. Fixed the sense of the sentence to be right. $\endgroup$
    – user107153
    Commented Jun 30, 2019 at 10:11
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Yes. Any substance emits and absorbs thermal radiation at its internal optically active frequencies. (A perfect black body emits and absorbs radiation at all frequencies.) As a result it will radiatively heat or cool until it is in thermal equilibrium with its surroundings. Other thermal mechanisms are convection and conduction. Note that phase transition from gas to liquid is essentially non-radiative.

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