Strange Clouds

Astronauts onboard the International Space Station have been observing electric blue "noctilucent" clouds from Earth-orbit.

They hover on the edge of space. Thin, wispy clouds, glowing electric blue. Some scientists think they're seeded by space dust. Others suspect they're a telltale sign of global warming.

They're called noctilucent or "night-shining" clouds (NLCs for short). And whatever causes them, they're lovely.

"In January 2003 we enjoyed outstanding views of these clouds above the southern hemisphere," said space station astronaut Don Pettit during a NASA TV broadcast in January 2003."We routinely see them when we're flying over Australia and the tip of South America."

Sky watchers on Earth have seen them, too, glowing in the night sky after sunset, although the view from Earth-orbit is better. Pettit estimated the height of the noctilucent clouds he saw at 80 to 100 km ... "literally on the fringes of space."

"Noctilucent clouds are a relatively new phenomenon," says Gary Thomas, a professor at the University of Colorado who studies NLCs. "They were first seen in 1885" about two years after the powerful eruption of Krakatoa in Indonesia, which hurled plumes of ash as high as 80 km into Earth's atmosphere.

Ash from the volcano caused such splendid sunsets that evening sky watching became a popular worldwide pastime. One sky watcher in particular, a Briton named T. W. Backhouse, noticed something odd. He stayed outside after the sun had set and, on some nights, saw wispy filaments glowing electric blue against the black sky. Noctilucent clouds. Scientists of the day figured the clouds were some curious manifestation of volcanic ash.

Eventually the ash settled and the vivid sunsets of Krakatoa faded. Yet the noctilucent clouds remained. "It's puzzling," says Thomas. "Noctilucent clouds have not only persisted, but also spread." A century ago the clouds were confined to latitudes above 50^o; you had to go to places like Scandinavia, Russia and Britain to see them. In recent years they have been sighted as far south as Utah and Colorado.