Never Heard Of These Before

On the left is a photo of noctilucent clouds seen over Northern Ireland on June 2, 2006.






Here's how NASA's website describes them (from a 2003 article):

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). And whatever causes them, they're lovely.

"Over the past few weeks we've been enjoying outstanding views of these clouds above the southern hemisphere," said space station astronaut Don Pettit during a NASA TV broadcast last month. "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 hurled plumes of volcanic ash as much as 80 km high in Earth's atmosphere.