Most radiative transfer models developed for the analysis of urban configurations consider a transparent urban atmosphere and gray sky conditions. The aim of the present work is to investigate these assumptions, focusing on the longwave domain. For this purpose, reference Line-by-Line Monte-Carlo calculations are first performed. A more efficient radiative transfer model based on the Finite Volume Method and SLW modeling is also developed to treat several street canyon configurations. The aspect ratio of the street canyon, a parameter frequently encountered in urban scenarios, is considered for the analysis. It is shown that, in most cases, real urban problems need to take into account a non-gray participating atmosphere within the streets, together with its spectral coupling with the non-gray sky downward radiation, to provide accurate net fluxes on urban surfaces and volumetric radiative powers in the urban atmosphere.