When looking on the sky its lower part looks lite blue instead blue like the rest of the sky. Why is it so?

  • 1
    $\begingroup$ I wouldn't close this as a duplicate of the proposed question; that one doesn't ask about (or explain) phenomena near the horizon. $\endgroup$ Jun 24, 2021 at 17:29
  • $\begingroup$ I agree. The proposed duplicate addresses similar principles but it is not the same as this question. $\endgroup$ Jun 27, 2021 at 8:38
  • $\begingroup$ The answer to this question is here physics.stackexchange.com/questions/561101/… and the answers there specifically discuss the whiteness of the sky closer to the horizon. $\endgroup$
    – ProfRob
    Jun 27, 2021 at 10:02

2 Answers 2


As you probably already know, the sky is blue due to scattering of light. Blue gets scattered the most1. The light which "comes" from the horizon has to travel the largest distance (compared to the light which comes from points in the sky closer to the sun), so the light is scattered more. As a consequence, some of the blue light is "removed" by scattering, making the light closer to the horizon appear more white.

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1 It is actually purple that gets scattered the most, see Why is the sky not purple?


When you look in different directions you look through different amounts of air (and different layers): the total distance scales as $\approx 1+\tan^2(\theta)$ where $\theta$ is the zenith angle.

That means that besides the Rayleigh light scattering that makes the sky blue, there will also be more Mie scattering along the path. This is amplified by the larger amount of water droplets, dust, pollen and other scattering particles in the lower layers of the atmosphere. They all scatter sunlight, and this adds to the light along the path. This scattering tends to be more whitish since it scatters all wavelengths roughly equally, unlike the Rayleigh scattering.

  • $\begingroup$ Even without aerosols, with Rayleigh-only scattering, the horizon would be white. $\endgroup$
    – Ruslan
    Jun 26, 2021 at 21:18

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