Wonder if anyone would know what is the average thermal emissivity value of air relative to blackbody at near room temperatures? I am looking for that magical number between 0 and 1. Looked everywhere but couldn't find the value.

I am trying to enumerate thermal losses when heating air inside an envelope made of Polyethylene with very high transmissivity and hence low emissivity. I am thinking that actual radiative thermal losses of air itself might play an important role with regard to the temperature of the heated air. I am completely neglecting convective and conductive heat loss at this stage. (I understand that these are actually major contributors to the thermal balance.)

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    $\begingroup$ gas phase radiation heat transfer is a complex topic that doesn't lend itself to a simple black body "emissivity" calculation. You might want to try physics.stackexchange.com ( not saying its off topic here though ). All that said assuming the air temp is below the melting temp of polyethylene I suspect radiation is negligible. $\endgroup$
    – agentp
    Jan 6 '18 at 16:28
  • $\begingroup$ Um, no. You aren't looking for a number between 0 and 1. You are looking for a theory of irradiance and radiant exitance that accommodates translucent materials and fluids with temperature gradients. In the sizes where this matters, gas stratification and convection need to be considered as well. The standard opaque surface emissivity model is a nonstarter. $\endgroup$
    – Phil Sweet
    Jan 6 '18 at 16:56
  • $\begingroup$ So there is no standard value for emissivity of air under normal atmospheric conditions vs say thickness of air layer? As agentp stated the temperature differences between air and surface that it exchanges thermal energy would be 50 to 100K at most, could I just consider the amount of thermal radiation from air as negligible? Thickness of air layer would be aprox 1m $\endgroup$ Jan 6 '18 at 17:02
  • $\begingroup$ This might be a good starting place for estimating the heat losses. courses.marlboro.edu/pluginfile.php/56965/course/section/57654/… $\endgroup$
    – Mark
    Jan 7 '18 at 0:05

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