The sky has gone - well... really weird. That's all I can say. Well... here's a picture:

http://s90.photobucket.com/user/jakeofthevale/media/2014-05-22_00001_zps387aab7a.jpg.html

Any help?

Have you made any recent changes to your video hardware or drivers?

Ah yes, this happens sometimes. Just save, quit the game and restart it.

I did so... nothing. I loaded a previous save... it got worse.
http://s90.photobucket.com/user/jakeofthevale/media/2014-05-22_00002_zpsaea9e932.jpg.html

Wow, never seen this before. Are your graphics card drivers up to date? What are your computer specs? What mods are you using?

And... it's gone. O_o

I think it may have been because I deactivated Climates of Tamriel and ELFX: Exteriors to stop the Battle for Whiterun CTD'ing when I left the city to go to the blockade - activating them seems to have solved it. Thanks for the help, though!