Why is the sky pink in the evening?

Sky usually comes in blue, grey or dark color. But, there are times when the sky appears pink. Why is that so? Specifically, the phenomenon comes about through the interaction of light with the dust and soot particles high up in the atmosphere.

 

When light hits a dust particle in air, it is scattered into various component wavelengths representative of the various colors present in light. Such scattering, when repeated over many dust and soot particles in a small volume, would result in a color change to the original light beam and, by extension, a change in the color of the sky.

 

In the evening over a city, air pollutants from cars would have built up to a higher concentration compared to the early hours of morning. Comprising nitrogen oxides, sulphur oxides, particulates and other organic compounds, these pollutants come in a particulate fraction as well as gaseous components. Particles in the particulate fraction comprise aerosols with differing size and shapes, collected in a size and shape distribution that, depending on its width, would determine the scattering effect on light rays, and the resulting color of the sky.

 

Specifically, if the size and shape distribution of the aerosols particles is large, light would be scattered and re-scattered in myriad ways such that the overall change in color of the sky would not be significant. On the other hand, if the aerosol’s size and shape distribution is narrow, light would be predominantly scattered in one way; thereby, leading to a distinct change in color of the sky resulting from the change in wavelength of light.

 

Overall, aerosol particles in air scatters light and results in a color change of the sky; thus, giving rise to the phenomenon of a pink sky in the evening. Depending on the size and shape distribution of the aerosol particles, the change in the wavelength of light could come in two guises: (i) over a wide wavelength range, or (ii) specific change in color. In general, narrow size and shape distribution in aerosol particles engenders a distinct color change, while a large size and shape distribution would usually result in scattering of light over a wide wavelength range that does not manifest in a color change in the sky.

 

Category: environment, physics, atmospheric science,

Tags: aerosol, size and shape distribution, wavelength, atmospheric scattering,

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