If I had to guess: - the static clouds are either flare-like textures, or effects; - the moving clouds seem to be cloud textures scrolling (tcMod scroll) on a curved patch; - realistically, the sun and the satellites seem to also be made of scrolling textures on patches. If that's not the case, they might be static textures on a func_rotating, perhaps mixed with a misc_skyportal; - I can't exactly figure out how, but the sky seems to be a mixture of a static night texture, a dawn / dusk shader which seems to show up twice per "day", and a day sky which covers the night sky, all of them properly timed to show up at specific phases (perhaps with an alphaGen wave w x y z shader?). I can't say if it's an actual skybox or rather patches in combination with a misc_skyportal. If I had to do it I'd probably choose the night sky as the skybox and the day sky as a dome-like patch. The real problem with such a day-night cycle is that the lighting is completely unaffected. It's less visible in this case because the facility is already artificially well lit, so you'd expect it to still be bright at night time as well. Rend2 would probably eliminate the problem, since it has a console command to activate a dynamic sun light which rotates, effectively creating the day and night lighting needed.