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NASA reports that rare, electric blue noctilucent clouds have reappeared over the South Pole, where the clouds are often spotted for five to ten days every year. NASA calls the clouds "a great geophysical light bulb" that are visible during the darkest nights.

The clouds were spotted by NASA's AIM spacecraft, which observed a "vast bank" of the clouds that began on November 20 and has expanded to blanket the entire continent, creating a rippling mass of particles that represent the highest clouds formed on earth. The clouds "glow" because of their altitude - they reflect light cast from a horizon we can't see from the ground. But what causes these clouds to form so high above the surface of the earth?

Last year, atmospheric scientists from Hampton University published a study revealing the discovery of "meteor smoke" in the clouds. When meteors get pulverized in the atmosphere, they leave behind a trail of tiny bits floating in the upper reaches of the atmosphere. It turns out that these microscopic "meteor clouds" provide the building blocks for noctilucent clouds - water molecules gather on the specs of dust, creating ice crystals.


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© NASA/AP
These polar mesospheric clouds in the Northern Hemisphere were shot by the Expedition 31 crew aboard the ISS in June of 2012
These specks of ice are incredibly small, since they form in the very highest reaches of the atmosphere - which is almost a vacuum. According to Space.com, the cloud particles can be up to 100 times smaller than the particles that make up a normal cloud:
These tiny ice crystals also explain how noctilucent clouds get their electric-blue color. Small particles typically scatter short wavelengths of light (blue) more than longer wavelengths (red). So from our perspective on the ground, when a beam of sunlight hits a noctilucent cloud, the scattered blue color is what we see.
Back when the clouds were first discovered, in the mid-19th century, you had to go deep into the Arctic or Antarctic landscape to see them. But, today, they're appearing more and more often above inhabited regions (for example, over Glasgow in the lead image).
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© NASA/AP
A 2008 NASA image of noctilucent clouds show the phenomena as the ISS passed over western Mongolia.
NASA explains that the increase in methane in our atmosphere is to blame. When the gas reaches 60 or 70 miles above earth, it oxidizes to create more water vapor - adding to to the noctilucent clouds. NASA's scientists describe the increase as a "canary in a coal mine" for climate change.

So keep an eye out for these luminous, ragged clouds - but keep in mind that while they're beautiful, they're also an ominous symbol of how we're changing not only the earth, but the atmosphere above it.