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 Earths surface, and are visible at high latitudes. They gleam blue or white when they catch the suns rays, even after the night has fallen on land. “Theyre beautiful,” says James Russell, an atmospheric scientist at Hampton University in Virginia. “Its hard to take your eyes off of them, because theyre 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. “Its 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.