A rare event turns the Grand Canyon into Cloud CityS

Over the weekend, rare atmospheric conditions gave rise to an even rarer scene: The Grand Canyon – that vast, gaping wonder of the natural world – filled to the brim with cold, dense, roiling fog. National Park officials have called the sight a "once in a lifetime" event. Looking at these photos, it's not hard to see why.

All images courtesy the National Park Service, photographed by Park Ranger Erin Whittaker, via The Grand Canyon National Park Facebook Page

A rare event turns the Grand Canyon into Cloud CityS

The Canyon came to be filled with fog thanks to what meteorologists call a "temperature inversion," a phenomenon whereby warm and cool air (which typically reside at lower and higher altitudes, respectively) swap places. According to the National Weather Service, the atmosphere's temperature profile is most prone to inversion during the winter, when long nights allow for air near the Earth's surface to become unusually cold:

A rare event turns the Grand Canyon into Cloud CityS

Once the sun goes down, the ground loses heat very quickly, and this cools the air that is in contact with the ground. However, since air is a very poor conductor of heat, the air just above the surface remains warm. Conditions that favor the development of a strong surface inversion are calm winds, clear skies, and long nights. Calm winds prevent warmer air above the surface from mixing down to the ground, and clear skies increase the rate of cooling at the Earth's surface.

A rare event turns the Grand Canyon into Cloud CityS

Those clear skies can make for a breathtaking view, if fog should somehow become trapped in the cold bottom layer of a particularly strong inversion. This is exactly what happened at the Grand Canyon twice over the course of the weekend, with freezing fog dominating at altitudes just below the canyon's outer rim. According to the NWS, "local topographical features can enhance the formation of inversions, especially in valley locations." The GC being a bit more than "a valley location," the inversion that formed –and the resulting vistas – wound up being rather extraordinary.

A rare event turns the Grand Canyon into Cloud CityS

Park Ranger Erin Whittaker, who captured the photos featured here, told MailOnline that temperature inversions happen once or twice a year on average, but rarely as dramatically as this, and almost never against such a perfectly blue sky.

For many, many more photos and continuing updates, visit the Grand Canyon National Park Facebook Page.