External Longwave Radiation (Sky temperature)

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Hello everyone,

I have a small doubt regarding the longwave radiational part of the surface energy balance equation. As per the documentation, E+ calculates external longwave radiation using the following equation: q"_LWR = q"_gnd + q"_sky + q"_air

In the case of a flat roof, we can ignore q"_gnd (ground), then q"_LWR = q"_sky + q"_air if q"_sky (= esigma(T_sky^4 - T_surf^4)) is the net thermal radiative flux between a horizontal surface and the sky (Walton 1983) and q"_air (= esigma(T_air^4 - T_surf^4)) is the net thermal radiative flux between a horizontal surface and the ambient air.

Then how do we define q"_sky or T_sky? Is it like T_sky is the effective temperature of a region above a certain height above ground and space? and T_air is the ambient air which is below that certain hight? Are there any criteria for that specific height?

Otherwise, it is redundant to also include a radiation term for the air (q"_air) if we consider q"_sky is the net thermal radiation from a horizontal surface to the sky.

Any help is highly appreciated! Thanks