The illustrious Don Pettit took this amazing picture of noctilucent clouds from space. This image is of some clouds over the arctic, glowing bright green in the middle of the night. Check out another picture of the lovely light and find out how it works, and the mystery surrounding it.
Noctilucent clouds are clouds that shine at night, when everything around them seems dark. They tend to show up towards the poles, and appear more often in the summer. The optics that make them glow are fairly simple. Anyone who's ever seen a sunset has noticed that, although they on the ground are in shadow, the clouds overhead are illuminated by the sun. The curve of the Earth blocks out the sunlight from reaching the ground, but the clouds above aren't blocked, and are still illuminated. The higher up things are, the longer they'll catch the light. Noctilucent clouds are so high up that they're illuminated, often shining an electric blue, long after everything around them is dark. They're extremely dramatic, especially when seen from space.
And there is a mystery surrounding these clouds. Why have they popped up so recently? The earliest actual note of them is in 1885, after the Krakatoa explosion. The dust in the air made sunsets so gorgeous that people stayed out to watch them, and some sky gazers noticed pale, high clouds that shone at night. People hadn't seen them before, and T.W. Backhouse, the first person to call attention to them, thought they might be some strange kind of volcano dust that glowed. Physics doesn't change, and there have always been clouds and the sun. Why did people first make note of them then?
Some people think that the clouds are the product of global warming. The heat is causing water vapor to drift higher before it condenses. Others say it's more pollutants and dust in the atmosphere, making more visible clouds form at higher levels. There's also the possibility that people had always seen noctilucent clouds, but that the eruption from it made them more dramatic. Since they've acquired a name, more attention has been called to them. In any case, they're very pretty. And a little eerie.