What color is the sky? Most people, without thinking about it, would answer blue. That is indeed the correct answer, but why is that? Why can’t it be green or yellow instead? The answer is more complex than you might think.

When discussing colors, we must always refer back to lights of different wavelengths. A specific wavelength gives a specific color that can be perceived by human eyes; for example, light with a wavelength of 450 nm gives a blue color while light with a wavelength of 650 nm gives a red color. What about sunlight? Sunlight appears to be white as it contains light of all wavelengths [1].

Light rays travel from the sun to Earth in a straight line. However, they can be intercepted, forced to abandon their initial trajectory and spread over all directions [2]. This phenomenon is known as Rayleigh scattering. Rayleigh scattering occurs when sunlight hits air molecules in the Earth’s atmosphere. This causes the charged protons and electrons in the air molecules to oscillate, from which electromagnetic radiation at the same frequency is emitted to all directions [2]. Blue light, having short wavelengths and high frequencies, is scattered much more strongly than any other light that has a longer wavelength; in fact, the intensity of the scattered light was found to be proportional to the fourth power of the frequency [1, 2]. When we look at the sky, we are seeing the scattered light, and therefore the sky should look bluish-purple. However, our eyes are less sensitive to violet light [1, 2], and therefore the sky appears blue throughout the day.

Hang on, during sunset, the sky appears red. Why is that the opposite of what was described? That’s because too much scattering of blue light has occurred. When the sun sets, it appears on the horizon. As we are probably looking directly at the sky in the vicinity of the sun on these occasions, it becomes the light rays which directly reach us that matter. During sunset (or sunrise), sunlight will have to travel a longer distance in the atmosphere than it had compared to when the sun was directly overhead at noon. This means blue light is scattered even more in the atmosphere, with the majority of it deflected to the directions away from us [3]. What can reach our eyes eventually becomes mostly the less scattered light with longer wavelengths. Since the light at those wavelengths appear red to us, the sky around the setting sun results in the familiar color of sunset red.

Another interesting fact is that the color of the sky is different depending on which planet you’re on. It turns out that if you’re on Mars, the sky appears reddish during the day and blue around the setting sun [4, 5], a complete inverse of what we’re used to on Earth. This is because Mars’ atmosphere is usually filled with dust, resulting in a different pattern of light scattering [4, 5].

Next time you go out on a summer’s day, take a moment to appreciate our blue sky as well as the science behind its coloring. As long as we seek it, there will always be beauty in nature.

References:

[1] Lee, B. (n.d.). Why is the sky blue?. Retrieved from https://www.hko.gov.hk/en/education/earth-science/optical-phenomena/00364-why-is-the-sky-blue.html

[2] Del Genio, A. D. (2003, April 7). Why is the sky blue?. Scientific American. Retrieved from https://www.scientificamerican.com/article/why-is-the-sky-blue/

[3] Met Office. (n.d.). Why is the sunset red?. Retrieved from https://www.metoffice.gov.uk/weather/learn-about/weather/optical-effects/why-is-the-sunset-red

[4] Stoller-Conrad, J. (2020, April 21). Why Is the Sky Blue?. Retrieved from https://spaceplace.nasa.gov/blue-sky/en/

[5] Siegel, E. (2018, May 17). Ask Ethan: Why Does Mars Have A Bright, Red Sky? Forbes. Retrieved from https://www.forbes.comstartswithabang/2018/03/17/ask-ethan-why-does-mars-have-a-bright-red-sky/?sh=1a25a8513453