Graphics bug here. I've seen this twice now. Restarting the game fixes the problem.

Instead of describing it, here are screenshots:
http://cloud.steampowered.com/ugc/612716583522710755/6D113706610BAC36F7ED4196E7A21800597D0152/
http://cloud.steampowered.com/ugc/612716583522709828/44BD1366BB0715F42B6FC4229334FC27065276D8/
http://cloud.steampowered.com/ugc/612716583522711752/8F3875CDB520FCA4AE29BE6700D02B636798F280/

As you can see, it tends to only effect the fire/smoke graphics. I've only seen it start in Windhelm, but it's only happened to me twice so this could be a complete coincidence.

Like I said, restarting the game fixes the problem, so this post is mostly a "hey Bethesda please fix this in the next patch" post, but if anyone knows how to fix it I would be appreciative.