What Causes Noctilucent Clouds?

Northern summer has arrived, and that means it's time to watch out for noctilucent clouds (NLCs). On some summer nights, just after sunset, NLCs snake across the sky glowing electric blue-white.

Noctilucent clouds are a mystery: They hover 80 km above Earth at the very edge of space. Some researchers believe they're a sign of global warming. Others say they're caused by space dust and rocket exhaust. Whatever they are, they're beautiful. Normally a high-latitude phenomenon, NLCs have been seen in recent years as far south as Colorado and Virginia.

Ash from the Indonesian volcano caused such splendid sunsets worldwide that evening sky watching became a popular past time. One sky watcher in particular, a German named T.W. Backhouse who is often credited with the discovery of noctilucent clouds, 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. Scientists of the day figured they 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 50o; 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 more than a year ago, while in orbit aboard the space station.

"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 km high." The mesosphere is not only very cold (-125 C), 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.

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.

Are NLCs a thermometer for climate change? A telltale sign of meteoroids? Or both? "So much about these clouds is speculative," says Thomas.

A NASA spacecraft scheduled for launch in 2006 will provide some answers. The Aeronomy of Ice in the Mesosphere satellite, or AIM for short, will orbit Earth at an altitude of 550 km. Although it's a small satellite, says Thomas, there are many sensors on board. AIM will take wide angle photos of NLCs, measure their temperatures and chemical abundances, monitor dusty aerosols, and count meteoroids raining down on Earth. "For the first time we'll be able to monitor all the crucial factors at once."

Meanwhile, all we can do is wait ... and watch. There's never been a better time to see noctilucent clouds. "During the summer months, look west perhaps 30 minutes to an hour after sunset when the Sun has dipped 6- to 16-degrees below the horizon," advises Thomas. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted an NLC. Observing sites north of 40 degrees latitude are favored.

FMI: www.nasa.gov