Time-lapse: Watch fog fill the Grand Canyon in rare weather event

The rare climatic event was caused by what meteorologists call a temperature inversion.

A cold, moist air mass settled into the canyon, eventually creating a 500-foot-thick "low stratus deck" of clouds, said Brian Klimowski, meteorologist in charge at the National Weather Service office in Flagstaff, Ariz.The phenomenon usually happens only once every few years.