Hi Shailan!
so you mean that radiance extracts the sky conditions such as sky clearness from the weather data not just altitude and longitude of the city?
That is what gendaylit is typically used for. Please check the manpage at GENDAYLIT :
" /Gendaylit/ produces a RADIANCE scene description based on an angular distribution of the daylight sources (direct+diffuse) for the given atmospheric conditions (direct and diffuse component of the solar radiation), date and local standard time. The default output is the radiance of the sun (direct) and the sky (diffus) integrated over the visible spectral range (380-780 nm). We have used the calculation of the sun's position and the ground brightness models which were programmed in /gensky. (...)"
/Cheers, Lars./
/