In this photo provided by the National Park Service, visitors to Mather Point on the South Rim of Grand Canyon National Park, in Ariz., view a rare weather phenomenon - a sea of thick clouds filling the canyon just below the rim, Thursday, Dec. 11, 2014. Cory Mottice of the National Weather Service said the weather event happens about once every several years, though the landmark was treated to one last year.
In this photo provided by the National Park Service, visitors to Mather Point on the South Rim of Grand Canyon National Park, in Ariz., view a rare weather phenomenon - a sea of thick clouds filling the canyon just below the rim, Thursday, Dec. 11, 2014. Cory Mottice of the National Weather Service said the weather event happens about once every several years, though the landmark was treated to one last year. Michael Quinn AP
In this photo provided by the National Park Service, visitors to Mather Point on the South Rim of Grand Canyon National Park, in Ariz., view a rare weather phenomenon - a sea of thick clouds filling the canyon just below the rim, Thursday, Dec. 11, 2014. Cory Mottice of the National Weather Service said the weather event happens about once every several years, though the landmark was treated to one last year. Michael Quinn AP