Meteor dust, global warming, and even rocket exhaust: they've all been suggested as causes of the clouds pictured above. The clouds are called polar mesopheric clouds, and they appear near each pole during its late spring and early summer, the sun illuminating them at late twilight from below the horizon (which is why they're also called "noctilucent" or "night-shining"). The International Space Station took this picture as it passed over the Tibetan Plateau about two weeks ago. Earlier this month, it took the first picture of mesopheric clouds from space as it traveled past Western Asia. The clouds can also be seen from the ground and flying planes.

Below, recent Pictures of the Day:

Image: NASA.