This summer, scientists have reported seeing a surprising number of night-shining clouds in the Northern Hemisphere. High in the sky, such noctilucent clouds remain aglow even after sundown. Typically, these sunlit wisps develop up high in polar skies. Making it as far south as Oklahoma and New Mexico — as some did this summer — is a rarity.
These clouds gleam blue or white as they catch the sun’s rays, even after sundown. “They’re beautiful,” observes James Russell. He’s an atmospheric scientist at Hampton University in Virginia. “It’s hard to take your eyes off of them,” he says, “because they’re so iridescent.” They float in the mesosphere, some 80 kilometers (50 miles) up, and normally at high latitudes.
The clouds form at −130° Celsius (-200° Fahrenheit), when water vapor condenses onto and then freezes around dust particles. This makes nanometer-sized ice crystals. What stood out in June was how wet the mesosphere was. “It’s record-setting,” says Lynn Harvey, an atmospheric scientist at the University of Colorado Boulder.
One possible explanation is that more moist air than usual rose this summertime. There may also have been a rise in methane within the upper atmosphere. It can enter chemical reactions that form water vapor.
NASA’s Earth Observatory released a satellite image that shows these noctilucent clouds covering the Arctic on June 12. It showed in easy-to-see white areas where sunlight was reflecting off of the clouds most effectively.
Russell and Harvey have been part of a team that’s been monitoring these clouds for 13 years. They’re trying to learn more about how these clouds form and whether they might reveal atmospheric changes due to global warming.
The scientists plan to use computers to model the conditions under which clouds form. They’re hoping to explain what seems to be a trend of these clouds to increasingly form outside of polar skies.