The color of the sky has been a topic of fascination and scientific study for centuries. To the naked eye, the sky often appears blue during the day and black or dark blue at night. However, the sky can take on a variety of colors depending on the time of day, weather conditions, location on Earth, and other factors. So can the sky ever appear pink? Let’s explore some quick answers to questions about why the sky can sometimes take on a pinkish hue.
Why does the sky appear blue?
The sky appears blue most of the time because of how sunlight interacts with the gases in Earth’s atmosphere. As white sunlight passes through the atmosphere, shorter wavelengths like blue and violet scatter more than longer wavelengths like red and orange. This scattering, called Rayleigh scattering, makes the sky look blue from the ground.
What causes a red sky at sunrise and sunset?
During sunrise and sunset, the sun is lower on the horizon and sunlight has to pass through more atmosphere. More of the blue light gets scattered away, allowing more red, orange, and yellow light to come through, causing dramatic red and orange sunrises and sunsets.
What conditions can make the sky look pink?
Several conditions can contribute to a pinkish sky:
- Dust or pollution particles in the air scatter red light more, creating vivid red sunrises and sunsets.
- High-altitude cirrus clouds can also scatter red light.
- After large volcanic eruptions, particles in the stratosphere can create blood-red sunsets.
- “Watermelon snow” is caused by cold-loving algae that turn snow pink, which can also tint the sky.
So while a truly pink sky is rare, atmospheric conditions can combine to give the sky a soft pinkish glow at times. Next, let’s go over some more detailed explanations.
Rayleigh Scattering: Why the Sky is Normally Blue
As mentioned above, the blueness of the sky during midday is caused by Rayleigh scattering. Here’s a more in-depth look at how this optical phenomenon works:
Sunlight is composed of a spectrum of wavelengths, including all the colors of the rainbow. Shorter wavelengths of violet and blue light are scattered more by the nitrogen, oxygen and other gases in Earth’s atmosphere. Longer red, orange and yellow wavelengths pass through more directly.
This wavelength-dependent scattering, discovered by Lord Rayleigh in the 19th century, scatters over 10 times more blue light than red. From the surface, we see the blue light scattered in all directions, making the sky look blue.
At sunrise and sunset, the sunlight passes through more atmosphere to reach our eyes. Much of the blue light has been scattered away, allowing more reddish hues to come through and create colorful sunrises and sunsets.
How Particle Size Affects Scattering
The amount of scattering depends on the wavelength of light and the size of particles it encounters. Smaller particles scatter short wavelengths more. This table compares the scattering effects on different wavelengths from atmospheric particles of different sizes:
| Particle Size | Blue Scattering | Red Scattering |
|---|---|---|
| Gas molecules | High | Low |
| Haze particles | Moderate | Low |
| Larger dust/water droplets | Minimal | Higher |
Nitrogen and oxygen molecules in the atmosphere are very small, scatter blue light well. Larger particles from pollution or dust scatter longer wavelengths more, allowing more red light to come through.
When Can the Sky Appear Pink?
As we’ve covered, a truly pink sky is rare. But at times, the interplay of atmospheric conditions can give parts of the sky a pinkish tint. Here are some examples:
Dusty or Polluted Air
Higher levels of dust, smoke, or pollution particles in the air scatter red light more readily, while blue light is scattered away. This can create vivid red sunrises and sunsets and a pinkish glow on the horizon. Locations with frequent wildfires often see pink-tinted skies.
High Cirrus Clouds
Wispy, high-altitude cirrus clouds are made of ice crystals. These crystals can refract and scatter red sunlight, while allowing blue light to pass through, giving parts of the sky a pink hue.
After Large Volcanic Eruptions
Huge volcanic eruptions like Krakatoa and Mt. Pinatubo spew tons of ash and gases into the upper atmosphere. Sulfate particles and droplets scatter red and orange light downwards, producing awe-inspiring blood-red sunsets and pink skies for months.
“Watermelon Snow” Algae
In alpine areas, colonies of cold-loving Chlamydomonas nivalis algae can turn snow bright pink. This “watermelon snow” contains a red pigment that can also tint the surrounding sky pink.
Other Rare Causes
- Red dust blown in from large desert storms
- Plankton blooms in oceans reflecting pink light into the atmosphere
- Artificial pink lighting during firework displays or laser shows
So while the sky doesn’t turn pink under normal conditions, occasional phenomena can create pinkish hues under the right circumstances.
Famous Examples of Pink Skies
Let’s look at some notable historic events that led to reddened or pink skies around the world:
Krakatoa Eruption Pink Sky – 1883
The catastrophic 1883 eruption of Krakatoa in Indonesia shot ash over 50 miles high. The widespread ash and gas caused vibrant red and purple sunsets lasting 3 years and made the moon appear blue-green.
New England’s Pink Sky – 1950
In 1950, strong forest fires in Canada transported smoke particles south into New England. Mixed with fog, the smoke particles scattered red light, causing the sky to turn a vivid pink color across areas of Vermont, New Hampshire and Maine for several nights.
Mt. Pinatubo’s Blood-Red Sky – 1991
When Mt. Pinatubo erupted in the Philippines in 1991, it injected vast amounts of sulfur dioxide gas into the stratosphere. This led to spectacular sunsets and vivid pink skies for over 2 years afterwards across the globe.
China’s Pink Sky – 2021
In 2021, dense dust storms blowing in from Mongolia and northwestern China engulfed areas like Beijing in a thick pinkish haze, tinting the skies pink throughout northern China for several days.
Western Wildfires – 2020 to present
Intense wildfires in western North America in recent years have filled the skies with smoke particles that scatter red light and create dramatic pink sunrises/sunsets. Areas like California, Oregon, and British Columbia have seen frequent pink-hued skies.
Photographic Examples of Pink Skies
Here are some stunning photographic examples of distinctly pink skies from around the world:
Pink Sky in Africa
This vibrant pink sky was caused by red dust blown in from the Sahara Desert.
NASA’s View of a Pink Sky
NASA satellites captured this view of a pink sky over northeastern China caused by a dense sandstorm.
California Wildfire Sunset
Smoky skies from California wildfires created this vivid pink sunset.
Pink Sky in Hawaii
A passing storm combined with dust from the Hawaiian island of Maui produced this softly pink sky.
Conclusion
While the sky is normally blue, occasional conditions can scatter light in ways that give parts of the sky a pinkish hue. Dust, pollution, volcanic eruptions, algae, and high clouds are some factors that can contribute to pink skies. But these phenomena are transient and the sky returns to its familiar blue after the effect passes. So while intriguing pink skies may occur for periods of time, the dominance of Rayleigh scattering means the natural color of the sky remains blue across most of the planet most of the time.