It's a phenomenon that's been noted throughout history: bright nights when you could read even though there was no illumination from the moon, candles or any other form of light.
Now, Canadian scientists believe they've unravelled the mystery — and there's even a possibility we could see more such nights because of our changing climate.
In a new study published in the journal Geophysical Research Letters, atmospheric scientists Gordon Shepherd and Young-Min Cho from York University in Toronto suggest rather than the sun or meteors being responsible, it's the result of an atmospheric phenomenon.
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The occurrence of these rare nights have been noted as far back as 113 BCE and been documented over centuries, with events in 1783 and 1908 in Europe, the authors say.
"The people who describe bright nights … talk about the fact that they go out when it should be dark, and they should have trouble finding their way around but they can see all their surroundings," Shepherd told CBC News.
"They can even read a book by this light. They don't seem to think of light coming from the sky, it's just that their world is just as bright as it is in the daytime."
With increased light pollution, however, we're losing the ability to catch this phenomenon for ourselves.
Waves in the sky
In 1991, Shepherd developed an instrument — the Wind Imaging Interferometer (WINDII) — that was launched onboard a NASA satellite that measured "airglow," an event caused when ultraviolet radiation from the sun separates oxygen molecules into oxygen atoms. As the atoms attempt to form into molecules once again, they release energy, seen as visible light, or airglow.
Typically, however, we don't notice this happening because it's below what is detectable by the human eye. But every so often, we do, in the form of those bright nights.
Shepherd and Cho wanted to figure out just what was causing airglow that became visible to the human eye: the bright nights. Using data from WINDII, the researchers analyzed two years, 1992 and 1996.
They found that wavelengths in the atmosphere along different lines of longitude were responsible.
"When the peaks of the different waves superimpose, then the intensities add up … by about a factor of 10, that's enough to make it visible to the human eye," Shepherd told CBC News.
And while this phenomenon has been recorded for centuries, it's actually not that common an occurrence. Their instrument detected it about seven per cent of the time. However, that's only because it was looking at the entire planet.
Since our view is much more limited, we would only see it about once a year, and particularly away from city lights where the sky is darker. Those living within 100 kilometres of cities, which are heavily light-polluted, likely wouldn't notice a difference.
Climate change influence
Shepherd also noted that these waves are generated by severe weather. Storms produce waves that get stronger as they move vertically, which in turn produce longitudinal waves.
"So if climate change increases the amount of severe weather, it could increase the frequency of bright nights as well," Shepherd said, though many living within 100 kilometres of a city may not notice.
And while Shepherd said that these bright nights aren't necessarily at the top of the list for concern when it comes to climate change, he said it's still part of the bigger picture.
"It isn't a big deal in terms of most of the things we worry about … bright nights would be lower on my list than higher temperatures," Shepherd said. "But it's part of it. It's part of the whole thing. Our environment is changing."