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Why is the sky blue at midday, and red at dawn and dusk?

By Andrew Sibley

An appreciative correspondent of Andrew Sibley’s article in the April 2023 issue of Creation magazine (Earth’s atmosphere, wonderfully designed for life)¹ asked for a more detailed explanation of the colour of the sky. The atmosphere wonderfully provides for life: oxygen to breathe, water vapour that gives essential rain, carbon dioxide that enables plant life and warms the planet, and the beauty of the colours of the sky.

Image from author

His query and the author’s reply follows:

Thank for the article on the atmosphere, in [Creation] vol. 45 no. 2. Please can you help me better understand why the sky appears blue? I have tried (in the past) to research this myself and have not found a clear explanation. The article says a sunset or sunrise appears red because the blue light is scattered and so we see predominantly red light. So far, so good. But then I am told that at noon, the blue light is scattered and so we see blue. But at sunset, the blue light is scattered and so we see red. Do you see my confusion? … I‘m still interested in reading a good explanation of why the sky appears blue.
Michael P

Dear Michael,

Thank you for your question. You ask for clarification as to why the sky is blue at midday, and turns yellow or red at sunset. This is what was written in the article:

“Why is the sky blue? Gases and particles in the air that are smaller than the wavelength of light cause it to be scattered (called Rayleigh scattering). The shorter wavelength of blue light is scattered more than longer wavelengths of light, thus more blue light reaches the observer.

Why are sunsets red? At sunset and sunrise, the light has to pass through a lot more atmosphere, and so practically all the blue light is scattered away from the observer, thus leaving more yellow and red light to reach the eye.”

I hope the following information makes it clearer. As noted above, sunlight passing through the atmosphere is scattered by Rayleigh Scattering. Such scattering occurs when the objects encountered, mainly nitrogen (N₂) and oxygen (O₂) molecules, are much smaller than the wavelength of light. As well as the particle size, the strength of scattering is dependent upon a number of other factors, including the length of travel of light through the atmosphere, and its wavelength.

Firstly, when the Sun is high in the sky, the sunlight passes through a relatively thin layer of atmosphere, and the sky is very bright, whereas as the Sun sets it passes through much more of the atmosphere so the strength of the light weakens.

CCA SA 3.0

Secondly, most of the atmosphere is composed of N₂ and O₂, with much smaller amounts of trace gases, and small amounts of particulate pollution on a clear day when the Sun is high in the sky. At dusk, more sulphate aerosols and other dust and particulate matter are encountered because of the shallow angle of the Sun’s rays towards the observer—i.e., the light passes through relatively more polluted atmosphere at dusk and dawn.

Thirdly, the amount of scattering of the different wavelengths of light, from red to violet, is inversely proportional to the fourth power of the wavelength. As red light has a longer wavelength (600–650 nm) it is scattered less than blue light (~450 nm); in fact, about five percent for red light and 20 percent for blue light, which is four times greater.

We should note as well, that violet light is actually scattered more than blue, but the atmosphere becomes increasingly opaque at the shorter wavelengths, so more violet light is completely filtered out, and blue remains. The blue colour is actually an average of all the light that is scattered, because even small amounts of red light get through to the observer, whereas pure sunlight without scattering would appear almost white (but it is too bright to look directly at the Sun).

At midday

This means that, at midday the Sun is bright and high in the sky, with the light passing through a small amount of relatively clean atmosphere. Blue light is scattered much more than red, so the sky appears light blue.

Sunset and sunrise

At sunset or sunrise, the sky often turns yellow, orange or red, and the Sun appears yellow. The Sun’s light has to pass through a longer stretch of atmosphere, perhaps as much as 30 times longer, and the blue light is scattered so much that it can no longer reach the observer. This leaves the yellow and red light, which is scattered less, to reach the observer. Atmospheric dust and sulphate aerosols from volcanic eruptions can also make for more colourful dawn and dusk vistas; light passing through the dust is forward scattered by Mie Scattering, thus causing a softer, but sometimes broader red glow near to the horizon.

Summary

As I pointed out in the Creation magazine article, the composition of Earth’s atmosphere is perfectly designed for life, with sufficient oxygen and water vapour to support human and animal respiration, and plant growth, while protecting us from harmful radiation. The atmosphere is also thin enough to allow visible light to pass through. All of this evidence of the beauty and wonder of creation speaks of the power and wisdom of the Creator.

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