GRAND CANYON, Ariz. (CNN) – It was a rare sight at the Grand Canyon Thursday. Time-lapse video from the National Park Service shows a sea of fog filling the canyon like water.
It’s caused by something called a “total cloud inversion.”
That happens when moist, cold air gets trapped in the canyon under a layer of warm air.
A lack of wind also means the clouds can stay low.
It doesn’t happen very often, typically just once every few years. But Thursday’s event is the second time it’s happened in the last six weeks.