Around this time every year in the higher latitudes, looking up to the sky after sunset might get you one of the most beautiful sights nature can provide. If you're lucky, you can catch these electric blue clouds looming dozens of miles up in the atmosphere, and they are downright incredible.
They're called noctilucent clouds and they only appear near the Arctic and Antarctic Circles during each hemisphere's respective summer season. These clouds hang right at the edge of the atmosphere — around fifty miles above the surface — and their incredibly high altitude (they are the highest-altitude clouds on Earth) allows them to glow an electric tint of blue even after sunset.
The clouds form when ice crystals in the extreme upper atmosphere, where temperatures are extremely cold, condense and form thin wispy clouds.
Humans haven't always seen noctilucent clouds — not that we know of, anyway. NASA notes that the first documented sighting of the phenomenon occurred a few years after the violent eruption of Indonesia's Krakatoa in 1883, and sightings of the rare clouds seem to be growing more frequent in recent years.
Aside from their apparent "birth" from the volcanic ash launched into the atmosphere by Krakatoa, experts say that the smoke and dust left behind by meteorites also appears to play a large role in noctilucent cloud formation.
"It turns out that meteoroids play an important role in the formation of NLCs," explains Hampton University Professor James Russell, the principal investigator of AIM. "Specks of debris from disintegrating meteors act as nucleating points where water molecules can gather and crystallize."
Another catalyst for noctilucent cloud formation are the condensation trails left behind by rockets as they lift off, as documented in the above picture taken after the launch of Space Shuttle Atlantis on June 8, 2007.
[Images via Wikimedia Commons, National Geographic, NASA, NASA via AP]