Why do (some) sunsets turn the sky pink?

📓 The short answer

Sunsets can turn the sky pink because of a phenomenon called scattering, in which the higher-wavelength blues and violets are scattered, leaving the lower-wavelength yellows, oranges, and reds to be more visible. Factors like pollution, humidity, time of year, and clouds can also affect the intensity and coloring of sunsets.

📚 The long answer

To answer this question, I'm going to break it down into three parts:

Why does the sky appear blue during the daytime?

All visible light appears white to us, but is actually comprised of a spectrum of different colors that each have their own distinct wavelengths. Red is on the lowest end of this spectrum and blue is on the highest end. (Side note: Violet is actually on the highest end of the light spectrum but we can't see violet nearly as well because of how our eyes work.)

When sunlight passes through the atmosphere, it is scattered in all directions due to all the gases and particles in the air. Blue light is more susceptible to this scattering because of its higher wavelength. This is why we see a blue sky most of the time.

Why does the sky appear red/pink/orange during sunset and sunrise?

During a sunrise and sunset, the Sun's light has to travel a greater distance to us. This presents more opportunities for the higher-wavelength blues and violets to be scattered, leaving lower-wavelength yellows, oranges, and reds to be more visible.

What causes variation in sunset and sunrise coloring and intensity?

Here are some factors that alter the appearance of sunsets and sunrises:

• Pollution: It's often said that pollution aerosols enhance the intensity of sunsets, but this is wrong. Pollution aerosols are relatively large and varied in size. This means the scattering they produce is not that wavelength-dependent. Put plainly, low-level pollution and dust tends to dull the vibrancy of sunsets (i.e. a polluted sky might produce a pink sunset when it would have otherwise produced a bright orange or red sunset with clean air).
• Humidity: In humid conditions, the water vapor in the air adheres to the particles in the air. The additional water bulks up the particle and essentially blocks light from passing through. This also reduces the bright red and orange intensity of a sunset, leading to more pink sunsets.
• Time of year: During winter, the sun sets on more of an angle which increases the amount of the time it takes to set. This allows for a longer sunset. Additionally, light is more reflective on ice crystals vs. water vapor which scatters the light more, leading to a more vibrant sunset.
• Clouds: Clouds, particularly those high enough in the sky to not block the light, reflect the light of a sunset leading to a more spectacular show.

Sources

📖 Book of the week

Free Play: Improvisation in Life and Art by Stephen Nachmanovitch

This book explores the role improvisation plays in both art and daily life. Nachmanovitch urges readers to cultivate spontaneity, curiosity, and collaboration in their creative endeavors. Drawing from the worlds of music, dance, and theater, Nachmanovitch provides actionable tips and inspiring insights for those seeking to tap into their own creative potential. With practical examples and relatable anecdotes, Free Play is an excellent resource for anyone looking to enhance their ability to improvise in all areas of life.

👀 Catch up on other curious questions

• How does lightning work?
• Why does the wind blow?
• Why do some stars appear to twinkle?

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