Well, it depends on what texture pack you download. Some texture packs don't change the sky, sun, moon, etc.
They're not texture packs, they're called shaders. I haven't found a shaders texture pack for you yet but there are some shaders mods: (js you need a really beefy computer to run this) http://minecraftsix.com/glsl-shaders-mod/
He is asking for clouds not shaders, realistic clouds can be added using texturepacks but you need to disable some functions from optifine