Night-shining ‘noctilucent’ clouds have crept south this summer

High in the sky, sunlit wisps remain aglow even after sundown. This summer, a surprising number of such noctilucent, or “night-shining,” clouds have been spotted in the Northern Hemisphere — and, unusually, as far south as Oklahoma and New Mexico, scientists report.

These clouds typically float in the mesosphere about 80 kilometers above Earth’s surface, and are visible at high latitudes. They gleam blue or white when they catch the sun’s rays, even after the night has fallen on land. “They’re beautiful,” says James Russell, an atmospheric scientist at Hampton University in Virginia. “It’s hard to take your eyes off of them, because they’re so iridescent.”

The clouds form when cold temperatures, around −130° Celsius, cause water vapor to condense and freeze around dust particles, making 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.

Possible explanations for that extra wetness include more moist air ascending in summertime than usual, or an increase in the atmosphere of methane, which can be oxidized to form water vapor.

A satellite image released by NASA’s Earth Observatory shows these noctilucent clouds covering the Arctic on June 12, with white areas showing where sunlight is reflected the most off the clouds and dark purple the least.

Russell, Harvey and colleagues have monitored these clouds for 13 years to learn more about how they form and whether they might reveal atmospheric changes due to global warming. The scientists plan to use computer models to simulate cloud formation under various conditions, in hopes of explaining the clouds’ southward stretch.