I recently played through The Elder Scrolls: Arena and was inspired to take some screenshots of the outdoor areas:

(original screenshot)
I used GIMP to do some layered effects:

(W.I.P.)
Eventually, I had removed everything in the original screenshot and made this drawing my own:

(finished work)
I was compelled by this ancient game's relatively advanced sky, complete with a real-time day/night cycle involving seperately moving sun, moons, and stars, color-fading, rain, snow, and dithered fog. Even HL2, with its outdoor areas does not feature such detailed life in the sky as Arena, which came out in 1994! Sure, we've got huge resolutions and anti-aliasing along with particle effects and physics galore, but...just take a stroll through TES: Oblivion and look at the sky for a few minutes and you will see how something so simple can really add to a realistic environment. I mean...even Quake 1 had moving skies...so...what's up?