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Noctilucent Clouds Spread and Mystify 227

Wired has a feature on noctilucent clouds, once seen only at high latitudes but increasingly visible now lower down the globe. The clouds result from ice crystals at altitudes of 50 miles, higher than five 9s of the atmosphere. What water ice is doing up there, in a region 100 million times drier than the Sahara desert, is only one of the mysteries associated with the clouds. They are a recent phenomenon: the first scientific description of noctilucent clouds was penned in 1885. For a time it was believed that the clouds were an effect resulting from the eruption of the Krakatoa volcano two years before. Since 2002, the clouds have been sighted — and photographed — as far south as Oregon, Colorado, and Utah. Some scientists believe that human-caused climate change is playing a role, but others doubt this. Two satellites are in orbit to study the clouds; NASA's AIM generated this day-by-day movie of clouds in the vicinity of the North Pole during 2008.

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