It's the HDR, it did that with Oblivion as well. As you look more towards the sky, you can see it better.
I know, but all that can be controlled through settings. The nuclear sky is very badly done HDR.
Example of what the mod All Natural does for Oblivion:
http://i275.photobucket.com/albums/jj317/Brumbek/All%20Natural%20Oblivion%20Mod/AN-Tints-IC-Comparison-Vanilla.jpg
http://i275.photobucket.com/albums/jj317/Brumbek/All%20Natural%20Oblivion%20Mod/AN-Tints-IC-Comparison-ANDefault.jpg