Published: June 23, 1999

Silvery-blue ice clouds known as noctilucent clouds that appear each year in the far northern and southern latitudes in the middle atmosphere were spotted over Colorado for the first time on June 22.

Some scientists believe the clouds form from increases in gaseous methane rising unimpeded through a natural "cold trap" located about eight miles above Earth’s surface, said Gary Thomas, a professor at the University of Colorado at Boulder’s Laboratory for Atmospheric and Space Physics. The rising methane may react with sunlight to form large quantities of water vapor that eventually freeze and circulate to the top of the atmosphere, facilitating noctilucent cloud formation.

Thomas, one of the world’s leading experts on noctilucent clouds, predicted in 1994 that the clouds would brighten by five to ten times and be visible over the continental United States by the 21st century. "This is a big event," he said. "While they are a beautiful phenomenon, these clouds may be a message from Mother Nature that we are upsetting the equilibrium of the atmosphere."

The clouds, which bask in the late sunlight some 50 miles over Earth’s surface, were observed on June 22 at about 9:30 p.m. by CU-Boulder meteorology Instructor Richard Keen while driving up Coal Creek Canyon southwest of Boulder. They also were spotted that evening by Utah State University physics Professor Mike Taylor from his home in Logan, Utah.

"I saw silvery clouds to the northwest that were distinctly brighter and higher than the other clouds," said Keen. The previous record for the southernmost sighting of these clouds in the continental United States was in North Dakota, some 500 miles to the north, said Keen.

Noctilucent cloud formation is likely hastened by increasing amounts of rising carbon dioxide from Earth. While CO2 is thought to contribute to global warming in the lower atmosphere, it ironically cools the middle and upper atmospheres, helping to facilitate noctilucent cloud formation.

For more information contact Thomas at (303) 492-7022, Keen at (303) 492-4440 or Jim Scott in the CU-Boulder News Services Office at (303) 492-3114.