OPOD - Why is the sky blue?

Last updated on September 16, 2023

OPOD - Why is the Sky Blue?

Have you ever wondered why the sky appears blue? It's a question that has intrigued scientists and poets alike. In this article, we will explore the fascinating phenomenon of why the sky is blue and delve into the scientific principles behind it.

Rayleigh Scattering: The Science Behind the Blue Sky

The answer lies in a process called Rayleigh scattering, named after Lord Rayleigh, who mathematically described it. When light passes through the Earth's atmosphere, it encounters tiny air molecules that act as scatterers. These molecules are about 1000 times smaller than visible light wavelengths. Interestingly, they scatter blue light approximately four times more strongly than longer wavelength red light.

The intensity of scattering is inversely proportional to the fourth power of the wavelength. This means that shorter wavelengths, such as blue light with a wavelength of 450nm, are scattered much more strongly than longer wavelengths like red light with a wavelength of 650nm. The reason for this wavelength dependence lies in the interaction between the frequencies associated with bound electrons within the atoms and the oscillating electric field of the light waves.

Violet Sky? Not Quite!

Although violet light has an even shorter wavelength than blue light and is scattered even more strongly, the sky does not appear violet. This is because sunlight is weaker in violet compared to blue, and our eyes are less sensitive to violet light. Therefore, the scattered blue light dominates our perception of the sky's color.

Dust, Aerosol, and Moisture: Altering the Sky's Color

Various factors can alter the color of the sky from its natural blue hue. Dust particles, aerosols, and moisture in the atmosphere can desaturate the blue color, giving the sky a milky white appearance. These scatterers are comparable in size or larger than light wavelengths and scatter all colors more or less equally. This phenomenon is known as Mie scattering.

Other Worlds: Exploring Different Skies

When it comes to other planets or moons, the sky's color can vary. Space artists often depict green skies, as seen in "Forbidden Planet." However, in the absence of dust or aerosols, all gases in the atmosphere act as Rayleigh scatterers, resulting in blue skies. Exceptions occur when gaseous components absorb light, such as chlorine or nitrogen dioxide, leading to a colored sky. Additionally, dust and aerosols present in the atmosphere can also influence the color of the sky. For example, the thin atmosphere of Mars, with its high dust content, gives the sky a pinkish hue.

The Science Behind the Blue Sky

To understand why the sky appears blue, we need to look at how sunlight interacts with the Earth's atmosphere. Sunlight consists of photons of all colors that pass through the air. When these photons encounter air molecules, a small proportion of them are scattered in every direction. Importantly, the scattered photons retain the same color and energy as the original sunlight.

However, blue photons are more strongly scattered compared to green and red photons. This differential scattering of light causes the sky to appear blue. Although the sky is not purely blue, it contains a small proportion of other scattered colors.

The Beauty and Variations of the Blue Sky

The blue sky is a sight that evokes joy and inspiration. It exhibits subtle variations in color throughout its expanse. Overhead, the sky appears darker, especially when observed from mountains or airplanes. As we gaze towards the horizon, the blue gradually pales, almost turning white.

The most captivating skies are often observed after heavy rain showers have cleansed the atmosphere of dust and aerosols. In these moments, the blue color seems to emanate solely from sunlight and pure air.

The Role of Air Molecules

Air molecules, primarily composed of nitrogen and oxygen, are even smaller than the wavelengths of visible light. Individually, they interact weakly with light. However, due to their immense numbers in the atmosphere, their collective effect becomes significant. These air molecules scatter sunlight in all directions, with blue light experiencing stronger scattering than longer wavelengths.

Curiously, the sky is not uniformly blue. As we venture closer to the horizon, we notice that the sky appears whiter. This phenomenon can be attributed to the longer path length of sunlight through the atmosphere near the horizon. Along this extended path, photons are scattered multiple times, causing the reds and greens to become as prominent as the blues, resulting in a white appearance.

It is worth noting that the near-horizon sky cannot become infinitely bright due to a certain atmospheric path length beyond which it becomes opaque. In fact, the sky often darkens slightly very close to the horizon for this reason.

In conclusion, the mesmerizing blue color of the sky is a result of Rayleigh scattering, where air molecules scatter blue light more strongly than other colors. Factors such as dust, aerosols, and moisture can alter the sky's color, desaturating it to a milky white. Understanding the science behind the blue sky adds another layer of appreciation to the beauty that surrounds us every day.

Air molecules are 1000X smaller than visible light wavelengths. They act as Rayleigh scatterers and scatter blue light ~4X more strongly than longer wavelength reds.

Rayleigh scattering:

When scattering particles are much smaller than the wavelength of light the process is known as Rayleigh scattering after Lord Rayleigh, John William Strutt, (1842 - 1919) who first described it mathematically. The scattering is inversely proportional to the fourth power of the wavelength. For example, blue light of 450nm wavelength is scattered 4.4X more strongly than 650nm red light. The wavelength dependence arises from the extent of coupling between the frequencies associated with bound electrons within the atoms and the oscillating electric field of the light waves. Coupling increases as the oscillation frequencies get more similar.

Rayleigh scattering requires that there be no coherence between the individual scatterers. In dense gases when molecules are closer together this condition is not satisfied and light is predominantly scattered forwards rather than in all directions. In dense gases and liquids another process can operate, Einstein-Smoluchowski scattering. Molecular motion and collisions produce exceedingly transient local density and refractive index fluctuations that act as scattering centres. The wavelength dependence is the same as for Rayleigh scattering.

Violet sky?

Violet is shorter wavelength than blue and is scattered more strongly. The sky is not violet because sunlight is weaker in violet compared to blue and the eye is less sensitive to it.

Dust, aerosol, moisture:

All these desaturate the sky's blue to give in the limit a milky white. These scatterers are of comparable size or larger than light wavelengths and they scatter all colours more or less equally (Mie scattering).

Other worlds:

Space artists love green skies, see that in "Forbidden Planet". Sadly, all gases act as Rayleigh scatterers and would give blue skies in the absence of dust or aerosol. The exceptions are when gaseous components absorb light, like chlorine or nitrogen dioxide, to give a coloured sky. Dust and aerosol also colour. The thin Martian atmosphere is pink from its high dust content.

"Why is the sky blue" in Atmospheric Optics.

Sunlight photons of all colours stream through the air. Its molecules scatter a tiny proportion in every direction. The scattered photons have the same colour and energy.

Blue photons are more strongly scattered than greens and reds.

The scattered light makes the sky appear blue. The sky is not pure blue as it also contains a small proportion of other scattered colours.

Blue skies make the heart soar and poets rush for quill and ink.

The blue dome has subtle variety. Overhead it is darker - noticeably so from mountains or airplanes. Near the horizon it pales almost to white. The best skies are after heavy rain has washed out dust and aerosol saying the colour is conjured from sunlight and pure air alone.

The sun's light is a mix of violet, blues, greens through to reds. Blues and violets have the shortest wavelengths, that of blue is ~450 nanometres or 0.45 thousandths of a mm.

Air molecules, mostly nitrogen and oxygen, are 1000X smaller still. They interact only very weakly with visible light but with their enormous numbers in the atmosphere we see the effects.

Air molecules individually scatter sunlight it into all directions. Blue light is scattered much more strongly than longer wavelengths. The air above us looks blue from that scattered sunlight.

At the risk of disillusioning poets, it is not a pure blue. All other colours are scattered as well but progressively more weakly towards red.

Why is the sky whiter near the horizon? Overhead there are only a few miles of dense atmosphere and sunlight photons are scattered once - if at all. Near the horizon the air path is 10X or more longer. Along it, photons are scattered several times and the reds and greens eventually become as strong as blues to yield white. The near horizon sky cannot become arbitrarily bright with the extra scattering because beyond a certain atmospheric path length it effectively becomes opaque. In fact the sky often darkens slightly very close to the horizon for this reason.

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Reference Atmospheric Optics

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