Global warming may have created a new type of cloud called noctilucent or "night shining" clouds (NLCs). They're thin and wispy and have an electric blue glow. "Over the past few weeks we've been enjoying outstanding views of these clouds above the southern hemisphere," says space station astronaut Don Pettit. "We routinely see them when we're flying over Australia and the tip of South America." He estimates they?re 50 to 62 miles above the Earth's surface, "literally on the fringes of space." You can see them from Earth as well, glowing in the night sky after sunset.
"Noctilucent clouds are a relatively new phenomenon," says Gary Thomas, who studies them. "They were first seen in 1885," about two years after the eruption of the Krakatoa volcano in Indonesia, which sent ash high into Earth's atmosphere.
Ash from that volcano caused such wonderful sunsets that watching them became a popular pastime all over the world. One sky watcher named T. W. Backhouse also noticed wispy filaments glowing electric blue against the black sky?NLCs. Scientists at that time thought they were caused by volcanic ash. But when the ash eventually settled, the NLCs remained, and now we're seeing even more of them.
"It's puzzling," says Thomas. "Noctilucent clouds have not only persisted, but also spread." A century ago, you had to go to places like Scandinavia, Russia and Britain to see them, but today they can be seen as far south as Utah and Colorado. "Although NLCs look like they're in space," says Thomas, "they're really inside Earth's atmosphere, in a layer called the mesosphere ranging from 50 to 85 kilometers high." The mesosphere is very cold and very dry?"one hundred million times dryer than air from the Sahara desert." NLCs are made of very tiny ice crystals, and sunlight scattered by these crystals gives them their blue color.
Clouds form when water molecules stick to dust in the atmosphere, which creates ice crystals. What scientists don't understand is how ice crystals can form in the dry mesosphere. Also, ordinary clouds get their dust from things like desert wind storms, but dust from the Earth can't get all the way up to the mesosphere.
"Krakatoa may have seeded the mesosphere with dust in 1883, but that doesn't explain the clouds we see now," says Thomas. "Perhaps the source is space itself." There is dust in outer space, which comes from debris cast off by passing comets and asteroids. And the water? "Upwelling winds in the summertime carry water vapor from the moist lower atmosphere toward the mesosphere," says Thomas. This is why NLCs appear in the summer, rather than the winter.
The reason for the recent increase in NLCs could be global warming. Noctilucent clouds were first seen during the Industrial Revolution, which was a time of rising greenhouse gas production. "Extreme cold is required to form ice in a dry environment like the mesosphere," says Thomas. Global warming creates a colder mesosphere because, while greenhouse gases warm Earth's surface, they actually lower temperatures in the high atmosphere. This is what causes blocks of ice to form in the mesosphere and fall out of the sky.
So much that's happening right now was predicted by Art Bell & Whitley Strieber in The Coming Global Superstorm?now being made into a major motion picture called "Tomorrow."
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