How Does the World's Most Beautiful Cloud Form?

LargeNucCloudNoctilucent cloud. Photo: Courtesy of NASA

When astronaut Chris Hadfield was asked last week what his favorite photo taken during the first three weeks of a stint on the International Space Station was, he answered simply.

"I love the beautiful pictures of the world," Hadfield said, "but for me, the one that was most significant was looking at the noctilucent clouds. These are clouds that you can barely see from the surface of the Earth. They're the highest clouds that exist—tiny ice particles way up in the mesosphere. And yet from orbit, as the sun rises, the light bounces off of those clouds, directly into our eyes—and we can see a part of the Earth's atmosphere that's basically invisible to people on the surface. To me, that's both beautiful—because of the colors and textures and ripples of it—but it's also really significant. It's a way to understand the changes in our atmosphere, and a way to understand exactly how our atmosphere interacts with the universe beyond."

What exactly did Hadfield mean by that last sentence, in which he says that the clouds can help us understand changes in our atmosphere and our relationship to the universe? I set off in search of the science behind noctilucent clouds for the answer.

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