Why is the Sky Blue?

Last updated on September 16, 2023

Why is the Sky Blue?

When we gaze up at the sky, we are often captivated by its beautiful blue hue. But have you ever wondered why the sky appears blue? In this article, we will delve deeper into the science behind this phenomenon and explore the factors that contribute to the color of the sky.

The Interaction of Light and Air Molecules

To understand why the sky is blue, we must first examine the interaction between light and air molecules. Sunlight is composed of a mixture of different colors, ranging from violet and blue to green and red. Each color corresponds to a specific wavelength, with blue light having the shortest wavelength of approximately 450 nanometers.

Air molecules, primarily nitrogen and oxygen, are significantly smaller than the wavelength of visible light. While these molecules only weakly interact with light individually, their immense numbers in the atmosphere collectively produce noticeable effects. When sunlight passes through the atmosphere, air molecules scatter the light in all directions.

Rayleigh Scattering and the Dominance of Blue Light

The scattering of sunlight by air molecules is known as Rayleigh scattering, named after Lord Rayleigh, who mathematically described this phenomenon. Rayleigh scattering is inversely proportional to the fourth power of the wavelength, meaning that shorter wavelengths, such as blue light, are scattered more strongly than longer wavelengths like red light.

As a result, when sunlight enters the atmosphere, blue light is scattered in all directions much more effectively than other colors. This scattering gives rise to the blue appearance of the sky. However, it is important to note that the sky is not a pure blue, as all other colors are also scattered to some extent, albeit progressively weaker towards the red end of the spectrum.

The Changing Color of the Sky

If you've ever observed the sky from different vantage points, you may have noticed variations in its color. For instance, when looking straight up from a mountain or an airplane, the sky appears darker blue compared to the horizon. This difference in color is due to the varying path lengths of sunlight through the atmosphere.

When sunlight passes through only a few miles of dense atmosphere directly overhead, photons are scattered minimally, if at all. In contrast, near the horizon, the path length of sunlight through the atmosphere is much longer, often ten times or more. Along this extended path, photons undergo multiple scattering events, causing the red and green wavelengths to become as strong as blue, resulting in a whitening effect near the horizon.

Factors Affecting Sky Color

While Rayleigh scattering is the primary reason for the blue color of the sky, other factors can influence its appearance. Dust, aerosols, and moisture in the atmosphere can alter the scattering process and desaturate the blue color, giving rise to a milky white sky. These scatterers are typically comparable in size or larger than the wavelengths of light, leading to a more uniform scattering of all colors (known as Mie scattering).

Beyond Earth: Skies on Other Worlds

The blue skies we are accustomed to on Earth are not exclusive to our planet. In fact, all gases act as Rayleigh scatterers and would produce blue skies in the absence of dust or aerosols. However, certain gaseous components in other planetary atmospheres can absorb light, resulting in different colored skies. For example, the thin atmosphere of Mars contains a high concentration of dust particles, giving rise to a pinkish hue.

In conclusion, the blue color of the sky is a result of Rayleigh scattering, where air molecules scatter shorter wavelengths of light more effectively than longer wavelengths. This scattering phenomenon creates a stunning blue dome above us. While the sky may appear uniformly blue at first glance, its color can vary due to factors such as path length and the presence of atmospheric particles. Understanding the science behind the color of the sky adds a new layer of appreciation for the beauty that surrounds us.

Why is the Sky Blue?

Beyond the freshly cut hedgerow a blue sky grades to near white towards the horizon. Image ©Les Cowley

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.

Rayleigh scattering:

When scatterering 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 come 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.

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Reference Atmospheric Optics

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