Night sky cooling, cooling by transmission or as 'heat sink'

  • Thread starter Elquery
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  • #1
Elquery
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Howdy all.
Regarding the phenomenon of radiative night sky cooling, where an object (such as a blackbody) on the surface of the planet cools via radiating energy into space: can the cooling be ascribed to the transmittance of the radiation through the atmosphere (as such a clearer night with less water vapor has a greater cooling effect) or can the atmosphere somehow be dubbed 'an absorption sink'?

At least one source has used the term heat sink: "Radiative cooling is a consequence of heat loss by longwave radiation emission toward the sky, where the sky can be treated as a heat sink for exterior surfaces of the buildings." - https://onlinelibrary.wiley.com/doi/full/10.1002/htj.21459

Another source used the word absorb as well as heat sink.

To me, it seems as though it is primarily the transmittance of the body radiation that determines the cooling effect (though certainly some is absorbed and subsequently diffusely re-emitted).
 

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  • #2
russ_watters
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Regarding the phenomenon of radiative night sky cooling, where an object (such as a blackbody) on the surface of the planet cools via radiating energy into space: can the cooling be ascribed to the transmittance of the radiation through the atmosphere (as such a clearer night with less water vapor has a greater cooling effect) or can the atmosphere somehow be dubbed 'an absorption sink'?
Any absorption and re-emission by the atmosphere will reduce the radiative cooling effect, because the atmosphere is warmer than space.
 
  • #3
Elquery
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Thank you Russ, that was my sense.
I'm trying to think in terms of: I am radiating at a certain rate, and whether I 'cool' is simply a matter of the net exchange with my environment, i.e. the environment is radiating back to me at a certain rate.

When I think of it this way, it makes sense. I confuse myself when I go further and consider the difference between transmittance of radiation through something vs lack of emittance due to an object being cold (the latter is how I've heard some describe radiative night sky cooling: that the night sky 'is cold' therefore the cold sort of 'absorbs' your radiation.)

Take the case of a big block of cold charcoal sitting in a room: I will radiate towards this charcoal and the charcoal will radiate towards me. If its colder than me, it'll radiate less to me than I to it. But the charcoal is not transmitting my energy in the way a clear night sky is (a gas). It is indeed absorbing my radiation. What affect does this difference have the dynamics of the two situations? I'm tempted to say that in the case of absorption, even if the object is very very cold, more energy will ultimately come back my way than in a situation of transmittance.
 
  • #4
sophiecentaur
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But the charcoal is not transmitting my energy in the way a clear night sky is (a gas).

You'll find that even the simplest models of this are actually quite involved.

Perhaps you could think of the atmosphere as a hemispherical charcoal shell with big holes in it, through which some of your radiated EM passes, straight into space. The rest of the EM warms up the shell, which would eventually reach an equilibrium temperature between you and space (0K). Clear sky is mostly holes and cloudy sky has no direct holes.

The Power radiated from you to the shell will be P = Aσ (T14 - T24) See this Hyperphysics link. (The A in the formula will be the shell material or the shell holes, depending.) The power from the shell to you and to space and the shell to space will be given by the same formula (assume Space is 0K temperature). Given time, the shell will get to your temperature - the speed will depend on the relative areas involved.
Depending on the ratio of the areas of hole to charcoal, your temperature will fall faster of slower as the charcoal takes time to warm up from your radiated heat..

But another issue is the supply of thermal Energy from inside you to your surface. The rate will depend on your core temperature and your insulation. It's obvious that insulating will increase your final temperature.

Other issues are the Energy from the Sun, of course and the fact that the atmosphere is distributed and absorbs energy selectively according to wavelength. It's the wavelength selectivity that gives rise to the Greenhouse Effect and affects the details of your original question.
 
  • #5
vis viva
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  • #6
Elquery
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Thanks for the responses, they're all helpful. Certainly an involved subject. I'll give that book a look.
 

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