WASHINGTON, June 19 (UPI) -- A strange weather phenomenon spawned high in the polar atmosphere was recently seen as far south as the contiguous United States.
On June 10, both on-the-ground observers and NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft spotted noctilucent clouds, or "night-shining" clouds. The high-altitude clouds form in the spring and summer as the warming lower atmosphere sucks heat from above.
The cooling upper atmosphere causes ice crystals to take hold of meteor dust and other high-altitude particles. Occasionally, enough ice crystals accumulate to form visible clouds -- long blue and white sparkling wisps.
Noctilucent clouds were first documented in the mid-1800s after the eruption of Indonesia's Krakatau volcano sent ash throughout the atmosphere. In addition to vivid sunsets, skywatchers observed thin, glowing clouds, high in the sky. Even after the ash subsided, observers continued to note the phenomenon.
NASA's AIM probe has been charting the appearances of night-shining clouds for several years now. The data suggest they're appearing earlier, more frequently and at lower latitudes -- which some scientists suggest is the result of the growing presence of greenhouse gases.
After the most recent noctilucent clouds were documented by AIM, scientists created a composite image of their presence -- as seen from space.
"The clouds appear in various shades of light blue to white, depending on the density of the ice particles," officials wrote. "The instrument measures albedo -- how much light is reflected back to space by the high-altitude clouds."