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Visitors to Grand Canyon National Park got a rare treat last Thursday when a temperature inversion filled the sprawling basin with a thick layer of clouds. Meteorologically speaking, an inversion occurs when the air closest to the ground is colder than the air above it, creating areas of dense fog and ground-hugging clouds. Such events are usually the result of oceanic warm fronts or displaced air masses in front of storm systems (such as the one that hit southern California last week). Michael Quinn of the National Park Service posted a timelapse of the action to YouTube. Check it out: