Some Really Strange Clouds

From SpaceWeather .com:

A few nights ago, sky watchers in Northern Ireland witnessed a vivid display of electric-blue noctilucent clouds. Also known as "NLCs," these clouds float through the outer reaches of Earth's atmosphere at the very edge 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. 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.

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.

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