What Determines the Colour of the Sky?

What Determines the Colour of the Sky?

by Jasmine Chan

Source: “Biorefinery: Organics into Crude Oil Alternatives.” Blue Sky Bio, www.blueskybio.co.uk/. 

INTRODUCTION TO THE SKY

We all know the basic term: sky. But what actually is the definition of the sky? It is the upper atmosphere of the Earth. [1] Have you ever wondered how the sky changes colours from hues of reddish-orange to blue? What changes the colour of the sky? In this article, I will be looking into how the colour changes in the sky, and how it is achieved.

BLUE SKY?

Since the sky is an atmosphere, it means that the sky should be transparent. How do we see the blue when we look up in the sky during the day? 

The blue sky humans see is due to the interaction between the sunlight and the Earth’s atmosphere, and how human eyes perceive the sunlight. When sunlight interacts with the atmosphere, a phenomenon known as Rayleigh scattering occurs. [2] Rayleigh scattering is the dispersion of electromagnetic radiation by particles that have a radius smaller than 1/10 of the wavelength of light. [3, 4] When sunlight hits molecules of gases within the atmosphere, light is scattered. [3] The shorter the wavelength of light, the more light is scattered. [2] As blue wavelengths are short, the sky would seem predominantly blue. [4] 

Fig.1 Visual Diagram of Rayleigh Scattering in the Sky.

Source: Wolfe, Jordan. “Cold Start to MLK Day.” Cardinal Weather Service, 20 Jan. 2020, cardinalwxservice.com/2020/01/20/cold-start-to-mlk-day/.

However, using the concept of ‘shorter the wavelength of light, the more light is scattered’ - doesn’t this mean that more violet wavelength should be scattered instead since violet has a shorter wavelength than blue?

Humans do not see individual wavelengths of light due to the structure of our eyes. [2] Human eyes have photoreceptors found in the retina known as rods and cones. Rods are used for greyscale vision and cones are used for coloured vision. [5] There are three types of cones: L, M and S. [6] Each type of cone detect a different range of wavelengths. [5] The L, M and S cones are sensitive to long wavelengths, middle wavelengths and short wavelengths respectively. [6] To obtain colours aside from red, green and blue, the messages sent to the brain integrate which allows humans to see more than these 3 colours. [5] 

Fig.2 Diagram of the Eye.

Source: “The Eye.” Blue Cone Monochromacy, www.blueconemonochromacy.org/how-the-eye-functions/. 

Blue wavelengths do not only stimulate S cones despite them having long wavelengths - but they actually also stimulate the L and M cones. Since the S cones are stimulated more compared to the other two, this means that the sky should be blue with a green hue if blue wavelengths are the only wavelength considered. If violet wavelengths are considered individually, the sky should be blue with a red hue. The S cones are more sensitive to violet wavelengths than the M cones, which is what forms the red hue. Since both blue and violet wavelengths are scattered simultaneously, the green and red hues in both case scenarios cancel out as they are opposite colours - forming the blue sky. [2] 

Fig.3 Visual Diagram of Rayleigh Scattering.

Source: Resnick, Brian. “Why Sunsets Are Better in the Winter.” Vox, 22 Nov. 2019, www.vox.com/science-and-health/2019/11/22/20970563/sunset-science-explained-rayleigh-scattering.

SUNSETS AND SUNRISES

Fig.3 Visual Diagram of Rayleigh Scattering During Sunrise/Sunset.

Source: Corfidi, Stephen F. “The Colors of Sunset and Twilight.” Storm Prediction Center, www.spc.noaa.gov/publications/corfidi/sunset/.

When the sun is low on the horizon, the sunlight has to pass through a thicker layer of the atmosphere compared to the sunlight travelling in the day. [7] As light has to travel through more particles, more blue and violet wavelengths are scattered away from the human eyes, leaving the longer wavelengths such as red, orange and yellow. [8]

BIBLIOGRAPHY

[1] “Sky.” Britannica, www.britannica.com/science/sky. 

[2] Koberlein, Brian. “Earth's Skies Are Violet, We Just See Them As Blue.” Forbes, 11 Jan. 2017, www.forbes.com/sites/briankoberlein/2017/01/11/earths-skies-are-violet-we-just-see-them-as-blue/?sh=577c63c5735f. 

[3] “Rayleigh Scattering.” Encyclopædia Britannica, www.britannica.com/science/Rayleigh-scattering. 

[4] “Blue Sky.” Blue Sky and Rayleigh Scattering, hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html. 

[5] “Rods and Cones.” Kazilek, 6 Jan. 2010, askabiologist.asu.edu/rods-and-cones. 

[6] Hurlbert, A., and Y. Ling. Colour Design (Second Edition). 2017. 

[7] Barrow, Courtney. “More than Just a Pretty Picture: The Science behind Stunning Sunsets.” AccuWeather, www.accuweather.com/en/weather-news/more-than-just-a-pretty-picture-the-science-behind-stunning-sunsets/360293. 

[8] “What Determines Sky's Colors At Sunrise And Sunset?” ScienceDaily, 15 Nov. 2007, www.sciencedaily.com/releases/2007/11/071108135522.htm.