Sunsets are often known for their beautiful orange and red hues. However, sometimes sunsets can also take on a pink or rose coloring. So what causes sunsets to turn pink instead of the typical warm shades? The pink color is due to the specific wavelengths of light that are scattered and absorbed in the atmosphere during sunset.
To understand what contributes to the pink color, it’s helpful to first review what makes regular sunsets red and orange. As the sun gets lower in the sky in the evening, its light passes through more of the atmosphere to reach our eyes. Shorter wavelength blue and violet light is scattered away, while the longer wavelengths like red and orange pass straight through. This gives sunsets their distinctive warm tones.
So in the case of pink sunsets, an additional factor is in play. The air contains more particulates than usual, often due to smoke or dust particles. These particles scatter more of the reddish light, allowing more of the light in the purple-red wavelength range to come through. To our eyes, this combination creates pink or rose-colored hues.
In this article, we’ll take a deeper look at:
What causes sunsets to be red and orange normally
– Rayleigh scattering affecting colors at sunrise and sunset
– Shorter blue/violet wavelengths scattered away
– Longer red/orange wavelengths pass straight through
How particulates scatter light to make pink sunsets
– Additional particles in the air from smoke or dust
– Particles scatter more reddish light
– Allows more purple-red wavelengths to be visible
A closer look at the visible light spectrum
– ROYGBIV – red, orange, yellow, green, blue, indigo, violet
– Shorter wavelengths of light are blue/violet
– Longer wavelengths of light are red/orange
– Pink light is a mix of red and purple wavelengths
Examples of conditions causing vivid pink sunsets
– Wildfire smoke
– Dust storms
– Volcanic eruptions ejecting particulates high in the atmosphere
– Pollution hazes
Let’s explore the science behind pink sunsets in more detail.
What Makes Regular Sunsets Red and Orange
Sunsets are known for their beautiful red, orange, pink, and yellow hues. But why do sunsets produce these warmer colors, compared to the blue sky we see during midday? The reason has to do with the scattering of different wavelengths of visible light through the atmosphere.
Rayleigh Scattering Affects Visible Light
The scattering of light by particles in the atmosphere is called Rayleigh scattering, after Lord Rayleigh who first described the mechanism in 1871. Rayleigh scattering is wavelength dependent – it scatters shorter wavelengths of light much more strongly than longer wavelengths.
In the visible light spectrum, blue and violet light have the shortest wavelengths. Meanwhile red and orange light have longer wavelengths. This means the short wavelength blue/violet light gets scattered very efficiently by air molecules and other tiny particles. The longer wavelength red/orange light passes straight through with little scattering.
Blue and Violet Light Scattered Away from View
During the day when the sun is high overhead, sunlight has a relatively short path through the atmosphere. At noon the sun’s light only passes through about 1 atmosphere (atm) worth of air.
The short wavelength blue light gets scattered everywhere by the molecules it encounters in the atmosphere. Part of this scattered blue light reaches our eyes, causing the sky to look blue.
In contrast, the long wavelength red/orange light goes straight through with minimal scattering. This unscattered sunlight appears white when it reaches our eyes.
Red and Orange Light Pass Straight Through
But as the sun gets lower in the sky near sunset, its light has to travel through more air to reach us. At sunset, sunlight passes through around 10 atm worth of air compared to 1 atm at noon.
That’s much more atmosphere for Rayleigh scattering to act on the light. By the time the sunlight reaches our eyes at sunset, virtually all the short wavelength blue/violet light has been scattered away. This leaves the longer wavelengths of red and orange to pass straight through, giving sunsets their distinctive warm colors.
How Particulates Cause Pink Sunsets
So Rayleigh scattering explains the red and orange hues we typically see at sunset. But what causes some sunsets to turn pink or rose-colored instead?
The key is having additional particulates in the air, especially high up in the atmosphere. These particles provide further scattering of light that emphasizes pinkish wavelengths to produce vibrant pink sunsets.
More Particles in the Air from Fires or Dust
On most clear days, the air doesn’t contain many large particles. Tiny nitrogen and oxygen molecules make up the majority of the atmosphere. But some conditions can add more solid particles like smoke, dust, ash, or pollution to the air. Sources of these extra particles include:
– Wildfire smoke from large fires
– Dust kicked up from major dust storms
– Volcanic eruptions ejecting ash into the upper atmosphere
– Air pollution haze from high density cities
These particles are much larger than individual air molecules. So they have a different scattering effect on light compared to Rayleigh scattering.
Particulates Preferentially Scatter Longer Wavelengths
While air molecules scatter short blue light very efficiently, particulate matter tends to scatter the longer wavelengths more. This includes red as well as intermediate wavelengths in the purple-red range.
With more particulates around at sunset, they scatter away more of the red light. This leaves the colors toward the purple/red end of the spectrum to come through. Our eyes perceive these combined wavelengths as pink or rose-colored hues.
Allows Purple-Red Wavelengths to Become Visible
In a typical sunset, wavelengths in the purple-red range are mostly scattered away like the blue light. Only the longer red wavelengths pass through the atmosphere.
But with extra particulates to also scatter more of the red light, it allows more visibility for the purple and pinkish tones. Combined with the remaining red wavelengths, it creates vivid pink and magenta colors we see in the sky during some sunsets.
A Closer Look at the Visible Light Spectrum
To fully appreciate how particle scattering produces pink sunsets, it helps to visualize the spectrum of visible light wavelengths.
ROYGBIV – The Colors of the Visible Spectrum
The visible light spectrum consists of wavelengths our eyes can detect. The colors of visible light are commonly remembered using the ROYGBIV acronym:
| Color | Wavelength (nm) |
| Red | ~700 |
| Orange | ~615 |
| Yellow | ~580 |
| Green | 500-565 |
| Blue | ~475 |
| Indigo | ~445 |
| Violet | ~400 |
As shown in the table, red light has the longest wavelength visible to our eyes, at around 700 nanometers. Blue and violet have the shortest wavelengths.
Short Wavelengths are Blue/Violet
Light on the short wavelength side of the spectrum appears blue or violet. Wavelengths ranging from about 400-500 nm are seen as various shades of blue and purple.
The extremely short wavelengths just beyond 400 nm are ultraviolet light. Though not visible, the scattering of UV rays is what causes the sky to look blue by scattering blue light in all directions.
Long Wavelengths are Red/Orange
On the long wavelength end, light appears red for wavelengths from about 615-700 nm. Light from 565-615 nm shows up as various shades of orange.
Interestingly, infrared light starts just beyond 700 nm. Though we can’t see it, we can feel infrared as heat. The warmth of sunlight on your skin is caused by absorption of infrared wavelengths.
Pink is a Mix of Red and Purple Wavelengths
Pink and magenta light result from a mix of the longer red wavelengths with the shorter purplish wavelengths. So when more particulate matter scatters away the orange-red light, it leaves the purples and reds to combine into pink tones in the sky.
Examples of Conditions Causing Vivid Pink Sunsets
Certain natural and man-made conditions can produce vivid pink and magenta sunsets. Here are some examples:
Wildfire Smoke
Smoke from large wildfires contains tons of particulate matter from ash and burnt vegetation. Prevailing winds often transport this smoke thousands of miles, creating hazy sunsets even far away from the fires themselves. The ash particles preferentially scatter the red light, allowing more pink and purple tones to shine through.
Some of the most vivid pink sunsets occur downwind of major forest fire complexes. The massive 2020 wildfires in the western United States led to striking pink and orange sunsets viewable across the entire country.
Dust Storms
Similar to wildfire smoke, large dust storms also introduce more solid particles into the air that can create vivid sunsets. Regions like the desert southwest United States occasionally experience giant dust storms or haboobs that spread particulate matter high into the atmosphere.
The fine dust particles effectively scatter the sunset light. If the dust reaches all the way to the stratosphere, it can scatter light globally for months after a major dust event.
Volcanic Eruptions
When volcanoes undergo major eruptions, they blast huge amounts of gaseous and particulate matter into the sky. The ash and sulfuric acid aerosols released into the stratosphere also serve as excellent scatterers of light.
Some of history’s most spectacular sunsets followed major volcanic eruptions. The powerful 1815 eruption of Mount Tambora in Indonesia led to purple and red sunsets across the globe for over a year.
Air Pollution Haze
Areas with high levels of air pollution often experience vivid sunsets. Pollutants like sulfates and nitrogen oxides emitted from factories and vehicles introduce lots of haze-producing particulates into the local atmosphere. These can scatter sunset light to produce stunning pink and orange sunsets.
Major urban areas are well-known for their beautiful smoggy sunsets. Cities like Los Angeles and Beijing often get vivid pink and red sunsets due to their high pollution levels. However, the particulates causing them are also a health hazard.
Conclusion
In summary, the pink and rose-colored hues sometimes seen in sunsets are caused by additional particles in the air. These particulates come from sources like smoke, dust, volcanic ash, and pollution. They preferentially scatter more of the red light from the setting sun, allowing purplish-red wavelengths to shine through. Combined with the remaining scattered red light, it produces the unique pink colors in dramatic sunsets.
While pink sunsets are beautiful to behold, some of the conditions that cause them come with health downsides like air pollution. However, others like clean wildfire smoke high in the atmosphere can make brilliant sunsets visible for thousands of miles. Whatever the cause, a vivid pink sunset is a stunning atmospheric phenomenon to witness.
