Firstly, let me say that this is a really cool idea! I actually had a similar plot going once, but the PCs managed to prevent the BBEG from getting the spell off. I'd love to hear how all this turns out.

The way I was planning to do it was that the heat on the edge of the spell's effect would be causing the snow just within the spell's borders to evaporate rapidly shortly after sunrise, causing a ring of mists during the day which blew away during the night. The BBEG had minions with blindsight to harass interlopers in the mists during the day shift and undead for the night shift. Maybe not as scientifically accurate (or even RAW accurate?) as some of the above ideas, but it creates a great dramatic tension when the PCs have to fight difficult to see/hit skirmishers in a low visibility setting.