http://www.denverpost.com/ci_5579483...Noctilucent clouds appear only at night, when their altitude - 50 miles up in the atmosphere - lets them catch sunlight no longer visible from Earth's surface, said James Russell, an atmospheric scientist at Hampton University in Virginia, the NASA mission's principal investigator.
That makes noctilucent - "night-shining" - clouds appear almost iridescent, he said. Most normal clouds are less than 10 miles up.
"They're very beautiful, they look very mysterious, but aside from all that, these clouds are changing in ways we don't understand," Russell said.
Researchers have many ideas about why noctilucent clouds may be growing in number and brightness, and most of them are related to global warming. [continued]