The sky is actually purple, but your eyes are lying to you
In short: Why does the horizon ignite in scarlet while the midday sky remains blue? The answer lies in a cosmic game of obstacle courses. As sunlight travels across the atmosphere, it undergoes a transformation that filters out the short waves, leaving only the long, vibrant colors to reach your eyes.
The phenomenon of the colorful sunset is driven by a process called Rayleigh scattering. When sunlight enters the Earth's atmosphere, it strikes molecules of oxygen and nitrogen. Because these molecules are roughly a thousand times smaller than the wavelengths of incoming light, they preferentially scatter the shortest wavelengths first. This is why the sky appears blue during the day; the blue and purple light is being bounced around in every direction by the air itself.
However, as the Sun approaches the horizon at sunset or sunrise, the light must travel through a significantly longer path of atmosphere to reach an observer on the ground. During this extended journey, the blue and purple wavelengths are scattered away so thoroughly that they effectively disappear from our line of sight. This leaves behind the longer wavelengths, which can pass through the atmosphere more easily, resulting in the vivid oranges and reds we associate with the end of the day.
Interestingly, the daytime sky would technically appear purple to humans if not for our specific biological limitations. Our eyes are most sensitive to the middle of the visible spectrum—the green range—which sits closer to blue than to purple. This biological bias masks the true color of the scattered light. Furthermore, the intensity of a sunset can be heightened by larger particles like dust and pollution. While these particles can create a 'muddled' palette, they can also produce strikingly pure colors depending on the atmospheric conditions and your specific perspective, such as looking down from above the atmosphere's boundary layer.
This means that a 'spectacular' sunset is often a matter of perspective and filtration. From an airplane above the boundary layer, where large particles like dust and pollution are trapped below, the colors can appear much more vivid than they do from the ground, where the atmosphere's contents provide a more complex, filtered view of the sun's radiation.
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Sources
- Summary by Intellect · source: Why Does the Sky Turn Red at Sunset?
- Also: Red Sky at Night: The Science of Sunsets
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