Fifty miles overhead, higher than any other formation in our planet's atmosphere, is where you'll find one of the most enigmatic classes of cloud ever observed. They're called noctilucent clouds, and they're some of the most poorly understood meteorological phenomena on Earth.
Noctilucent clouds (aka polar mesospheric clouds) are notoriously difficult to view from the ground, and are only visible once the Sun has dipped below the horizon. As a result, one of the best places to observe them is actually from outer space. Here's your chance to witness this remarkable sight as the astronauts do.
This particular image [click here for hi-res] was captured from aboard the International Space Station as it passed over the Tibetan Plateau on June 13, and it is absolutely stunning. The icy noctilucent formations are the wispy, stream-like formations coursing their way across the image in a layer of Earth's atmosphere known as the mesosphere. These clouds are literally looming at the very edge of space.
Look closely, and you'll see a thin band of orange dividing the cerulean sky from the dark outline of the planet below. This is the stratosphere, the lowest layer of visible atmosphere.
According to NASA, "the exact cause for the formation of polar mesospheric clouds is still debated," though rocket exhaust, dust from meteors and volcanic activity have all been fingered as potential explanations. The fact that noctilucent clouds have become easier and easier to spot from Earth since their discovery at the end of the 19th century has even led some to hypothesize that their formation could be catalyzed by warming global temperatures:
Top image via NASA, noctilucent clouds from Earth photographed via Wikimedia Commons