Rare night-shining clouds that glow across the edge of space tend to appear near Earth's poles. But since the turn of the century, these silvery clouds have become more frequent sights over lower latitudes, including southern Canada and the northern United States, new research finds.
Also known as noctilucent clouds, night-shining clouds are the highest clouds in Earth's atmosphere. They hover around 50 miles (80 kilometers) above the surface of the planet - high enough to reflect sunlight long after sunset. The wispy clouds were first officially documented in 1885. Since 2007, scientists have been monitoring the phenomenon near the poles with NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite.
"AIM and other research has shown that in order for the clouds to form, three things are needed: very cold temperatures, water vapor and meteoric dust," study author James Russell, an atmospheric and planetary scientist at Hampton University in Virginia, said in a statement from NASA. "The meteoric dust provides sites that the water vapor can cling to until the cold temperatures cause water ice to form." [In Images: Mysterious Night-Shining Clouds]