marcush. wrote:the atmosphere has an emmissivity of around 0,81(average) so the error against a standardised black /grey emitter at 0,9 to 1 is present but not as severe as a shiny metal surface which has a emissivity of .05 !
The main contributor to emissivity in atmosphere is water vapor (around 0,66) so you need to know moisture content of the air you are looking at as it is really the main variable ....
Atmospheric emissivity is a little tricky. Emissivity is based on an infinitely thick dimension –or until the emitting material become opaque. The atmosphere is neither infinitely thick nor opaque. So the .81 is through the entire typical atmosphere.
Since N2 and O2 have zero emissivity, the average concentrations of H2O vapor, CO2, CH4 and O3 ¬ -the principle emitting gases in air- are used for the .81 figure. Surface troposphere air is typically different from the atmosphere as a whole.