Once rare high-altitude clouds, called noctilucent clouds, are not only becoming increasingly common in the late evening and early morning skies, but also teaching atmospheric scientists much about the upper atmosphere and how climate change is affecting it. Noctilucent clouds form in the Earth's mesosphere (the next atmospheric level above the stratosphere) at an elevation of about 50 miles. To put that in perspective, that's very close to the "edge of space" reached by billionaires Sir Richard Branson and Jeff Bezos in their space flights in July 2021. That high, well above 99.99 of the Earth's atmosphere, it's amazing that clouds can form at all, and those that do form are too tenuous to be seen in daylight. To spot them, you have to wait until the sun is far enough below the horizon, but for the upper atmosphere to still be bathed in sunlight. "You can only see them after sunset or before sunrise," says Cora Randall, an atmospheric scientist at the University of Colorado's Laboratory of Atmospheric and Space Physics and its department of Atmospheric and Oceanic Sciences. "They appear to glow at night."
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