Mysterious Clouds that Shine at Night

PureInsight | March 10, 2003

[] Thin, wispy clouds hover on the edge of space, glowing electric blue. They are called noctilucent or "night-shining" clouds (NLCs for short). The photograph above was taken by astronaut Don Pettit aboard the International Space Station.

"Over the past few weeks we've been enjoying outstanding views of these clouds above the southern hemisphere," said Pettit during a NASA TV broadcast last month. "We routinely see them when we're flying over Australia and the tip of South America." Pettit estimated the height of the noctilucent clouds he saw at 50 to 62 miles (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 kilometers 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 German 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 degrees; 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.

Astronaut Don Pettit is a long-time noctilucent cloud-watcher. As a staff scientist at the Los Alamos National Laboratory between 1984 and 1996, he studied noctilucent clouds seeded by high-flying sounding rockets. "Seeing these kinds of clouds [from space] ... is certainly a joy for us on the ISS," he said on NASA TV.

"Although NLCs look like they're in space," continues Thomas, "they're really inside Earth's atmosphere, in a layer called the mesosphere ranging from 50 to 85 kilometers high." The mesosphere is not only very cold (-193 Fahrenheit, or -125 Celsius), but also very dry--"one hundred million times dryer than air from the Sahara desert." Nevertheless, NLCs are made of water. The clouds consist of tiny ice crystals about the size of particles in cigarette smoke. Sunlight scattered by these crystals gives the clouds their characteristic blue color.

How ice crystals form in the arid mesosphere is the essential mystery of noctilucent clouds.

Ice crystals in clouds need two things to grow: water molecules and something for those molecules to stick to--dust, for example. Water gathering on dust to form droplets or ice crystals is a process called nucleation. It happens all the time in ordinary clouds.

Ordinary clouds, which are relatively close to Earth, get their dust from sources like desert wind storms. It's hard to waft wind-blown dust all the way up to the mesosphere, however. "Krakatoa may have seeded the mesosphere with dust in 1883, but that doesn't explain the clouds we see now," notes Thomas. "Perhaps," he speculates, "the source is space itself."

Are NLCs a thermometer for climate change? A unusual sign of meteoroids? Or both? "So much about these clouds is speculative," says Thomas. A NASA spacecraft scheduled for launch in 2006 should provide some answers.


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