Tourists and park staff were treated to a rare event on Thursday as, for the second time in as many years, clouds filled the Grand Canyon.

According to Brian Klimowski, meteorologist in charge at the National Weather Service in Flagstaff, Arizona, the phenomenon, called a cloud inversion, happens when a cloud band forms between a lower layer of dense, cool air and a higher layer of relatively warmer air. Recent rains and long, calm nights allowed a cloud deck, about 500 feet thick, to form just below the canyon rim.

Klimowski, who said he had a “front row seat” to last year’s inversion, said this kind of inversion is more common in other areas, but seeing two in two years at the Grand Canyon is uncommon.

“It is a relatively rare event for it to spill into the Grand Canyon. It happens once or twice a decade, but it’s absolutely spectacular,” he said.

While from the rim it looks like one could walk across the top of the canyon, those on the canyon floor looking up would see a low bank of clouds overhead.

As the air warms, the clouds will disappear, possibly not returning for many years.

Total cloud inversion seen from Yaki Point: South Rim of #grandcanyon National Park, Dec.11, 2014. pic.twitter.com/p1tE8Txi3d

— Grand Canyon NPS (@GrandCanyonNPS) December 12, 2014