Last Thursday, visitors to the Grand Canyon in Arizona witnessed a spectacular event: the enormous valley filled up completely with fog. Natasha Greiling of Smithsonian explains that this event is known as a "total cloud inversion." That's when cold air becomes trapped in the bottom of the canyon beneath a heavy layer of warm air. This occurs once every few years at the Grand Canyon during unusual conditions.
Fortunately, the National Park Service staff who work at the canyon were ready. They took some amazing photos, as well as a time-lapse video of the event, which you can see below. It looks like the canyon is filling up from the bottom.
-via Ace of Spades HQ