The noctilucent or "night-shining" clouds are at an altitude of 47 to 53 miles (76 to 85 km), where meteors and bright aurora lights are not uncommon and the atmosphere gives way to the blackness of space. The clouds remain a scientifically baffling phenomenon more than 120 years after their discovery."It's lovely," said Gary Thomas, an atmospheric scientist at the University of Colorado after looking at a photo taken from the space station. "And it shows just how high these clouds really are – at the very edge of space."
The clouds form at dizzying heights where the air is one hundred million times drier than the Sahara. By contrast, the common high-altitude cirrus clouds only reach heights of 11 miles (18 km) up.
"We have a fairly good idea that the water vapor from below gets transported upwards," Thomas told SPACE.com. "That is in essence the fuel."
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