I call this “frost fog” but that’s probably not the right name for it. It’s frost that melts off early as the sun rises, and becomes a cold, dense, low-lying fog.
Many mornings when I would fly from Baltimore to Chicago (airline hub to airline hub) my flights would take me over the mountains where I now live, and I’d see the Appalachian mountains with puddles of clouds in the valleys. I don’t understand why this happens; it’s obviously not just a question of altitude, it’s got to have something to do with winds and shelter and the presence of water. But it’s surprisingly beautiful, quite often.
I can throw my bedroom window open and this is what I see: