Why does the sky turn pink at sunset?
The short answer
Sunsets can turn the sky pink because of a phenomenon called Rayleigh scattering, where the shorter-wavelength blues and violets are scattered, leaving the longer-wavelength yellows, oranges, and reds to be more visible. Factors like atmospheric pollution, humidity, time of year, and clouds can also affect the intensity and coloring of sunsets. This is why we often see a pink sky at sunset or sunrise.
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 light has the longest wavelength in the visible spectrum, while violet has the shortest. (Side note: Violet is actually on the shortest end of the light spectrum but we can't see violet nearly as well because of how our eyes work .)
Image by NASA is licensed under the public domain.
When sunlight passes through the Earth's atmosphere, it is scattered in all directions due to all the gases and particles in the air. Shorter wavelengths, like blue light, are more susceptible to this scattering, which 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 through the atmosphere to reach us. This presents more opportunities for the shorter-wavelength blues and violets to be scattered, leaving longer-wavelength yellows, oranges, and reds to be more visible.
Image by NASA is licensed under the public domain.
What causes variation in sunset and sunrise coloring and intensity?
Here are some factors that alter the appearance of pink sunsets and sunrises:
Pollution: It's often said that aerosols as a result of pollution enhance the intensity of sunsets, but this is wrong. Pollution aerosols 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 to pass through. This also reduces the bright red and orange intensity of a sunset, leading to more pink skies and 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 compared to water vapor, which scatters the light more, leading to a more vibrant sunset. This can cause pink skies at night or during cold mornings.
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. Pink clouds at sunset are often seen when the sun is low and its rays pass through a thicker layer of the atmosphere, amplifying the colors.
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Sources
Corfidi, S. F. (2014, September). The Colors of Sunset and Twilight. NOAA/NWS Storm Prediction Center. Retrieved April 22, 2023, from https://www.spc.noaa.gov/publications/corfidi/sunset/
Madaan, S. (2022, July 21). What Causes Pink Sunsets? Earth Eclipse. Retrieved April 22, 2023, from https://eartheclipse.com/science/geography/what-causes-pink-sunsets.html
NASA. (2022, August 29). Why Is the Sky Blue? NASA Space Place. Retrieved April 22, 2023, from https://spaceplace.nasa.gov/blue-sky/en
NBC Chicago. (2022, December 26). Yes, Sunsets Really Are Better in the Winter. Here's Why. NBC Chicago. Retrieved April 22, 2023, from https://www.nbcchicago.com/weather/yes-sunsets-really-are-better-in-the-winter-heres-why/
Resnick, B. (2019, November 22). Why sunsets are better in the winter. Vox. Retrieved April 22, 2023, from https://www.vox.com/science-and-health/2019/11/22/20970563/sunset-science-explained-rayleigh-scattering
University of Wisconsin–Madison. (2007, November 6). Curiosities: What determines the colors of the sky at sunrise and sunset? Retrieved April 22, 2023, from https://news.wisc.edu/curiosities-what-determines-the-colors-of-the-sky-at-sunrise-and-sunset/
