They've looked at clouds from both sides now
Have you ever looked to the west about half an hour after a summer sunset and seen wispy, luminescent clouds stretch pale blue tendrils across the sky?No? Then you need to get out more, and move north if possible.
These spectacular clouds--called noctilucent clouds--are actually on the very edges of Earth's atmosphere, almost in outer space itself. Astronauts on the International Space Station have seen them from above (clink the link to see photos), and those at 40 degrees latitude or more on Earth can see them routinely from below.
We live at about 39.5 degrees latitude, and I know I've seen them many times. I just thought they were clouds. I didn't realize they were "space clouds," or as I've decided to call them, "space-cicles."
Noctilucent clouds are formed of ice crystals, which are more common in the upper atmosphere now, due to two factors: 1) global warming: while greenhouse gases cook the lower atmosphere (the part we live in, unfortunately), they cool the upper atmosphere; 2) higher levels of space dust, as a result of no-one-knows-what.
Whatever the reason, they sure are purty.
