Create a background sphere, this will be the atmosphere and lighting for the sky (set it to render order 4, then create a clouds layer- this can be a plane, it just has to overlay the atmosphere and fade out in the distance (or make it a sphere/sky cube, whatever suits you) and set it to render order 3, then create the horizon (mountains/trees/buildings/etc) and set it to render order 2. Only things you need to update are the atmosphere/lighting and cloud layers. Former would probably be rotation along some axis, latter would usually just be the texture shifting and its alpha changing depending on 'time of day'.
Sorry, bit of a messy response.
Sorry, bit of a messy response.