The color of the sky is a topic that has fascinated humans since the dawn of civilization. The sky can take on many different hues depending on the time of day, weather conditions, and location on Earth. But the notion that the sky is actually purple is an intriguing idea that challenges our assumptions about color.
The Science of Sky Color
During the day, the sky appears blue because of how sunlight interacts with the gases in Earth’s atmosphere. As white sunlight passes through the air, shorter wavelengths like blue light are scattered more than longer wavelengths. This scattering, called Rayleigh scattering, makes the sky look blue from the ground.
But at sunrise and sunset, the light must travel farther through the atmosphere to reach our eyes. More of the blue light is scattered away, allowing more red, orange, and yellow light to come through. This gives the sky the vivid colors we associate with sunrises and sunsets.
So if the sky really isn’t blue, what causes some people to insist it’s purple? There are a few possible explanations.
The Purkinje Effect
One theory involves the way our eyes perceive color differently in bright versus dim lighting. This phenomenon is called the Purkinje effect after its discoverer Jan Evangelista Purkinje.
In bright light, our eyes are more sensitive to blue/green colors. But as light dims at dusk, our eyes become more sensitive to reddish/purple hues. Because of this shift, an object that appears blueish in daylight can take on a more purple cast at twilight.
So just as the setting Sun turns the sky reddish-orange to our eyes, the bright midday sky could look purplish if we were adapted to dim light. The Purkinje effect may cause some people to perceive the sky as more violet than blue.
Differences in Color Perception
Another factor is that people’s eyes and brains can interpret color differently. About 1 in 100 people have some form of color vision deficiency that affects their perception of certain shades.
For example, someone with tritanopia has trouble discriminating between blue and yellow. They may see the sky as a more purple hue rather than blue since their blue-yellow channel is impaired. Other types of anomalous trichromacy can also shift blue toward purple.
Moreover, even people with normal color vision don’t always agree on what counts as blue versus purple. There are no strict boundaries between spectral colors, so distinctions between blue and purple are somewhat arbitrary. The interval between these colors is a continuum of shades that differ based on individual perception.
Cultural and Linguistic Factors
The way cultures classify and name colors also affects perceptions of the sky’s color. Different languages divide the color spectrum differently. For instance, in Japanese culture there traditionally was no distinct word for blue – what English speakers call blue was traditionally described as a shade of ao, a word that covers blue and green.
Similarly, Ancient Greek made no distinction between blue and darker shades like purple. The word kyaneos was used for dark blues, violets, and even black. So to the Ancient Greek eye, the daytime sky may have contained more hints of purple than our modern English perception accounts for.
Here’s a table comparing some languages’ primary terms for blue/purple hues:
|Primary Term(s) for Blue/Purple
These linguistic divisions can shape whether the sky is even perceived as a distinct “blue” versus a more nuanced array of color gradations.
A Trick of the Eyes?
Given all these factors – lighting conditions, individual color perception, cultural semantics – is the notion of a purple sky just an optical illusion or trick of the eyes?
In a sense, yes. The actual color electromagnetic waves scattering in a clear daytime sky measure closer to blue than purple on the color spectrum. So technically, it appears blue by scientific measurement.
Yet our perception is not objective like a spectrometer. Human vision and interpretation introduces many variables that make “true” color more complex. In the end, the sky’s color depends on the beholder’s visual system.
While the sky itself may not contain much purple pigment, some combination of shifting light, individual perception, and cultural constructs can elicit purple sensations under the right circumstances. In that sense, although the sky is objectively blue, it can subjectively appear purple at times.
Famous Examples of “Purple Sky”
Many artists, photographers, filmmakers, and writers over the years have deployed the metaphor of a purple sky for visual impact or symbolic meaning. Here are some iconic examples that creatively depict the sky as purple:
- Vincent van Gogh – Van Gogh’s nightscape painting The Starry Night exaggerated the sky’s color with swirling strokes of blue, violet, and yellow.
- Prince – The album and film Purple Rain used a purple sky to represent euphoria and atmosphere. It became Prince’s signature color.
- The Matrix – In the Wachowskis’ sci-fi trilogy, the Matrix’s ominous simulated sky was tinted greenish purple to denote its artificial, constructed reality.
- Aurora borealis – The northern lights often mix purples with their shimmering greens, reds, and blues when solar particles interact with the atmosphere.
- Pollution – Haze pollution like LA’s smog can turn vivid sunsets deep orange, red and purple as light scatters through particles.
While not scientifically accurate, purple skies allow artists to evoke the mystical, surreal, ominous, or transcendent. They take creative license to say something about humanity’s relationship with nature using visual symbolism.
Purple Sky Sightings: Real or Imagined?
With all this discussion of purple skies as metaphor, are there ever real documented cases of the sky inexplicably turning purple?
There are in fact some eyewitness reports over the years of short-lived purple tinted skies anomalous to the weather conditions:
- Florida, 2002 – Residents across Florida reported seeing an eerie bluish purple sky one evening despite clear conditions.
- California, 2015 – Southern Californians were astonished to see a deep purple glow before sunset with no clouds or rain.
- Virginia, 2016 – In parts of Virginia, people spotted a brief purple/pink glow localized in the sky with no storm.
- England, 2018 – A purple sky appeared at dinnertime around Essex, UK, fading 15 minutes later. No explanation was found.
What could account for these spontaneous purple skies with no inclement weather? Some meteorologists conjecture they result from certain rare configurations of light refraction, or glimpses of high altitude auroras normally only visible much farther north. But these momentary purple phenomena still largely remain mysteries.
Mythology of Purple Skies
Purple skies have crept into mythology and folklore as harbingers of the supernatural:
- In Norse myth, a violet sky heralded the arrival of Fimbulwinter and Ragnarok.
- European fairy tales describe purple skies when magical beings use their powers.
- Some Native American legends tell of purple clouds appearing before visions and spiritual messengers arrive.
While these tales of purple skies clearly serve symbolic purposes, they imply people across cultures and eras have likely witnessed occasional purple-hued atmospheres they found ominous or significant.
Purple Skies in Sci-Fi and Fantasy
Imaginary purple skies are a fixture in many works of science fiction and fantasy. Some examples include:
- In the Star Wars universe, many planets like Coruscant have perpetually violet skies due to artificial weather control.
- The planet Exodus in Robert Heinlein’s story Universe featured vivid purple clouds.
- In Stephen King’s Dark Tower fantasy series, Thunderclap’s permanent purple sky resulted from technological cataclysm.
- Octarine, the Discworld books’ eighth fantasy color representing magic, is described as a fluorescent purple.
These futuristic or fantastical violet skies tap into associations people have with the mystical, unnatural quality of rare purple-hued skies in the real world.
So in review, while the normal daytime sky is objectively blue in terms of physics, sunrises and sunsets can shift toward red/orange thanks to ray scattering. And under the right environmental and perceptual circumstances, hints of purple may also emerge.
Factors like the Purkinje effect, individual color perception, and cultural color categorization all affect whether someone labels the sky “blue” or “purple.” And artists have long used purple skies as visual symbols for the fantastical, dreamlike, and transcendent.
Rare eyewitness accounts of unexplained purple skies do occasionally surface, showing this perceptual phenomenon still surprises people today. So while the typical sky remains blue, shades of violet can manifest under select conditions – reminding us our vision and reality are open to individual interpretation.