According to the national park’s Facebook page, the fog was caused by something called a ‘temperature inversion’. This happens when warm air acts as a lid to seal cool air near the ground. That means all pollution and fog is trapped and unable to rise.
Ranger Erin Whittaker told MailOnline that temperature inversions happen on average once or twice a year, but never producing such a picturesque full inversion.
As the DM(UK) and the slightly-pre-Thanksgiving National Park video below make clear, inversions are not impossibly rare. But this drastic (and photogenic) an example is a rare treasure.
Maybe November 29th was a good day after all.