Why Is the Sky Blue at Day and Red at Night?

I sometimes feel like the things we see often are the ones we rarely question. We are so used to seeing blue skies that we don’t ask ourselves why it isn’t green or pink. When we see a beautiful sunset, we are fascinated by its aesthetics. We probably don’t wonder why this ball of fire, which was yellow at noon, suddenly appears red to us. So, why is the sky blue at day and red at night?

A brief introduction to the electromagnetic spectrum of solar radiation

To understand why the sky appears blue, we must first comprehend how sunrays are formed and interact with the atmosphere. The energy of the sun travels in different wavelengths to our earth. As I remembered from my remote sensing course at university, a high frequency means a short wavelength. Simply because the distance between the respective wave crests is short. The short wavelengths are highly energetic, while the longer wavelengths transport less energy. These radiation waves form the so-called electromagnetic spectrum. The sun emits all sorts of wavelengths, some of them as X-rays or radio waves are wavelengths our eyes can´t perceive. However, the sun mainly releases wavelengths in the visible spectrum, as light we can see. ¹

Why is the sky blue?

The colour of the sky is determined by how sunlight reaches the earth. ² As you already know, the electromagnetic spectrum consists of different wavelengths and so does the visible part of light. The visible part of sunlight forms of different wavelengths that create different colours. What wavelengths are scattered and thus which colors we perceive depends on the consistency of the atmosphere. Our atmosphere consists of molecules such as Nitrogen, Oxygen and Carbon dioxide. Because these molecules primarily scatter blue wavelengths, the sky appears blue. In this sense, it is only a question of light scattering which colours we perceive. The effect that some wavelengths are scattered more than others is called Raleigh Scattering. ³

Why are sunsets red?

At sunrise or sunset, the sunlight has to travel a longer path through the atmosphere than at midday. Consequently, light interacts with more molecules that are in the air. Therefore, the blue light is scattered even more. It goes in different directions before reaching us. The long wavelengths of yellow, red and orange light can pass most of the particles in the atmosphere and makes up our beautiful sunset. ⁴ In other words, when the blue light is scattered by the atmosphere earlier than during the day because of the longer distance, red, orange and yellow light remains that can reach.

Conclusion

It depends on the way the sunlight is scattered in which colour we perceive the sky. During the day, the blue light is scattered in the atmosphere and the sky appears blue. At sunrise or sunset, the path of light through the atmosphere is longer. The blue light is scattered more, so that only red, orange or yellow light can reach us.

If you are interested in learning more about light scattering, check out my post about the formation of rainbows.

References:

1. Gunn, A., s.a. How much of the electromagnetic spectrum does the Sun emit? BBC Science Focus. Available at: https://www.sciencefocus.com/science/how-much-of-the-electromagnetic-spectrum-does-the-sun-emit/ [18.12.2022].
2. NASA Space Place, 2020. Why Is the Sky Blue? Youtube. Available at: https://www.youtube.com/watch?v=ehUIlhKhzDA [18.12.2022].
3. Owens, B., s.a. What makes the sky blue? Royal Museums Greenwich. Available at: https://www.rmg.co.uk/stories/topics/why-sky-blue [18.12.2022].
4. Cosmoknowledge. 2020. Why Are Sunsets Red? Youtube. Available at:  https://www.youtube.com/watch?v=fxRzHYhgi4s [18.12.2022].