It’s not uncommon for people to glance up at the sky and see streaks or swaths of pink and reddish hues blended amongst the usual blues. This phenomena is quite normal and has a simple scientific explanation behind it. While a pink sky is not rare per se, it tends only to occur at specific times of day when the sun is low on the horizon. Let’s explore when and why we can experience these colorful sunrises and sunsets.
What Causes a Pink Sky?
The pink and red colors we sometimes see in the sky are caused by the scattering of sunlight. This scattering is influenced by the particles and gases that make up our atmosphere. Here’s a quick overview of how it works:
- Sunlight contains all colors of the visible spectrum. When it passes unimpeded through clear air, we see the sky as blue.
- As sunlight nears the horizon, it passes through more atmosphere and intersects more air molecules and particulates.
- These particles scatter the short wavelengths of blue light, allowing more long red and pink wavelengths to pass through.
- The more particulates in the air, the more blue light is scattered away, causing the sky to transition to pink and red hues.
In essence, a pink sky occurs when there are enough air particles present to scatter away the blue light but still allow plenty of red light to reach your eyes.
When Does a Pink Sky Tend to Appear?
So when are ideal conditions present for creating vivid pink skies? There are two main times:
Sunrise and Sunset
The most common appearance of pink skies is around sunrise and sunset. This is because the sun is low on the horizon, so its light passes through more atmosphere and intersects more particles to scatter away blue light.
Some key factors that contribute to pink sunrises and sunsets:
- More dust and humidity near the horizon
- Longer path length through the atmosphere
- Light scattering at a low angle
Stunning pink sunrises and sunsets require just the right amount of cloud cover and dust or humidity to scatter away some blue light while still letting the warm hues through.
Large Forest Fires
The second most common cause of pink skies is large forest fires. The smoke, soot, and ash particles emitted from fires can scatter light and create vivid reddish sunsets across broad regions downwind.
Some key factors with fires:
- Abundant small particulates that effectively scatter light
- Smoke plumes that can reach high altitudes
- Carbon compounds in smoke that preferentially scatter blue light
Notable recent examples of widespread pink skies from forest fires include the 2020 West Coast fire season and the 2019-2020 Australian bushfires.
Is a Pink Sky Rare?
While a pink sky may seem unusual, it is not considered a rare event. Here are some points on the frequency of pink skies:
- Pink sunrises and sunsets occur regularly around the world, depending on local conditions.
- Vivid red and pink sunsets are more common in areas with more dust, humidity, and air pollution.
- Large smoke plumes from forest fires can create regional pink sky events every few years.
- Truly vibrant pink skies may only happen a few times per month or year in a given location.
- But some degree of pink tones mixed with orange and blue is common at sunrise and sunset.
So in summary, glimpsing some pink hues at dawn or dusk is not rare, but witnessing a dramatically pink sky is a less frequent treat.
Let’s examine the science behind pink skies in a bit more detail. We’ll look at the optics of scattering, the role of atmospheric particulates, and how color perceptions work.
The primary mechanism creating pink skies is Rayleigh scattering. This refers to the scattering of light off particles smaller than its wavelength.
Some key Rayleigh scattering traits:
- Most effective on the shorter blue wavelengths
- Scattered light is polarized perpendicular to the scattering plane
- Scattering intensity varies as 1/wavelength4
- Is responsible for the blue color of clear skies
At sunrise/sunset, the reddish hues we see are the remaining unscattered light after the blue end has been removed from the spectrum by Rayleigh scattering.
A secondary effect is Mie scattering – the scattering of light off larger particles like dust and water droplets.
Some characteristics of Mie scattering:
- Most significant when particle size is close to the wavelength
- Scattered light is mostly forward-directed
- Responsible for the white appearance of clouds
Mie scattering contributes by removing additional blue light, enhancing the reddish hues. It also creates brighter sunsets by reflecting more light forward into your eyes.
Aerosols and Particulates
The actual particles doing the Rayleigh and Mie scattering consist of:
- Nitrogen and oxygen molecules
- Water droplets
- Dust, soot, ash, and pollution
- Smoke from fires
More particulates in the air generally create more vivid sunrises and sunsets. But too much pollution can entirely block and diffuse sunlight, creating dull grey skies. There is a sweet spot between enough particles to scatter light effectively but not so many as to obstruct it entirely.
Perception of Color
Finally, the actual range of colors we perceive results from the response curves of our eye’s cone photoreceptor cells combined with color processing in the brain.
Key color perception factors:
- L, M and S cones respond preferentially to long, medium and short wavelengths
- Cone response curves overlap, allowing a range of perceived hues
- The brain combines and processes cone signals to produce color sensations
- Color appearance also depends on surrounding colors and lighting context
So the vivid reds and pinks we see at sunrise and sunset depend on both the spectra of light entering our eyes and how our visual system interprets those signals.
The frequency and intensity of pink sky sightings can vary significantly by location. Here are some geographic factors that influence pink sky potential:
|Region||Pink sky potential||Contributing factors|
|Urban areas||Moderate to high||Air pollution provides particulates for scattering|
|Forest fire zones||Episodically high||Smoke plumes scatter light when large fires occur|
|Coastal regions||Moderate||Typically more humidity provides scattering from water droplets|
|Arid climates||High||More airborne dust increases particulate density|
|High latitudes||Low||Less optimal sun angles at sunrise/sunset|
|Mountain regions||High||Enhanced scattering at high altitudes|
As the table shows, regions with more particulates, humidity, and pollution tend to have greater potential for vivid pink sky sightings. But other factors like latitude, elevation, and seasonal climate cycles can also have significant effects.
Records of stunning pink skies have inspired people for centuries. Here are some historical highlights:
- Ancient Greek poets like Homer referenced rosy-fingered dawns.
- Vivid red sunsets after the 1815 Tambora eruption inspired artists and writers.
- Pink sunsets featured in 19th century landscape paintings by Frederic Edwin Church.
- Impressionist artists like Monet and Renoir used pink tones to convey mood.
- Pastoral poets romanticized reddish sunsets and dawns on the countryside.
- Post-impressionists like van Gogh incorporated dramatic pink skies.
- Photographers chase vivid pink tones for eye-catching images.
So the ephemeral beauty of pink skies has long fascinated and inspired many. Scientists can now explain the optics behind these wonders of nature.
Folklore and Mythology
In addition to their scientific cause, pink skies have spawned many folk beliefs and superstitions over history. Some examples across cultures include:
- “Red sky at night, sailor’s delight. Red sky in morning, sailor take warning.”
- A red sunset was thought to signal the next day would be sunny and fair.
- Some Native American tribes believed red mornings were lucky.
- In China, a red sky is considered a sign of prosperity.
- Hindu mythology links red dawns to joy and the victory of good over evil.
- Pliny the Elder stated war would occur within three years of blood-red sunsets.
- An old English rhyme states “Pinks skies and sunsets, 24 hours wet.”
While most of these sayings are not scientifically accurate, they show how pink skies have inspired fascination and lore worldwide since ancient times.
Photographing Pink Skies
Capturing stunning photos of pink skies and sunsets takes skill and the right equipment. Here are some tips for photography:
- Use a DSLR camera with manual settings and RAW file format
- Choose a wide angle and zoom lens to capture broad vistas
- Get a sturdy tripod for long exposure shots
- Pick interesting foreground elements like mountains or buildings
- Arrive early and scout the location beforehand
- Bracket exposures to handle high dynamic range
- Wait for the sun to dip below the horizon for best color and contrast
- Adjust white balance cooler to enhance pinks and reds
- Avoid including the sun itself in your images
With practice, you can learn how to capture gorgeous pink sky moments for posterity.
While pink skies occur regularly around sunrises and sunsets, witnessing a particularly vivid pink sky is a special treat. The interplay of scattering optics, atmospheric particulates, and perceptual processes creates this magical sight. Photographers love capturing images showcasing nature’s canvas of color.
So next time you spot a rosy glow spreading above the horizon, take a moment to appreciate the natural beauty at hand. The science behind it may be understood, but an awe-inspiring pink sky will always be a wondrous sight to behold.