Why Is The Sky Blue?
QUICK ANSWER
The sky appears blue because of Rayleigh scattering, where air molecules in the atmosphere scatter shorter blue wavelengths of sunlight much more than longer red wavelengths. As sunlight travels through air, blue light bounces around in all directions, reaching your eyes from every part of the sky.
The blue sky is one of the most familiar sights on Earth, but the explanation behind it is one of physics' more elegant stories. Sunlight contains all colors of the rainbow, but air doesn't treat them equally. Tiny air molecules scatter blue light far more than red light, painting the daytime sky blue and the sunset sky orange and red. The same physics explains both at the same time.
What is Rayleigh scattering?
Rayleigh scattering happens when light hits particles smaller than its wavelength, like the molecules in Earth's atmosphere (nitrogen and oxygen). The scattering intensity is inversely proportional to the fourth power of wavelength, which means shorter wavelengths scatter much more than longer ones. Blue light (about 450 nm) scatters about ten times more than red light (about 700 nm) when passing through the same amount of air. Lord Rayleigh first described this in the 1870s, explaining why the sky is blue and the sun appears yellow.
Why isn't the sky violet then?
Violet light scatters even more than blue light, so technically the sky should appear violet. Two factors explain why we see blue instead. First, sunlight contains less violet than blue energy to begin with. Second, human eyes are far more sensitive to blue than to violet because of how our color receptors are tuned. The combination of these two effects shifts the perceived sky color from violet to blue. If our eyes worked differently, we'd see a violet sky on the same physical principle.
Why are sunsets red and orange?
At sunset, sunlight travels through much more atmosphere to reach your eye, because the sun is near the horizon and the light passes through a long horizontal slice of air. Along this long path, almost all the blue light scatters away in other directions before reaching you, leaving only the longer wavelengths (red and orange) to travel through to your eye. The same Rayleigh scattering that makes the overhead sky blue makes the sunset sky red. The redder the sunset, the more atmospheric particles are scattering light.
What about clouds and white skies?
Clouds appear white because they contain water droplets and ice crystals much larger than light wavelengths. Larger particles scatter all wavelengths roughly equally (Mie scattering), not preferentially scattering blue like air molecules do. This results in white scattered light. Hazy or smoggy days can make the sky appear whitish or gray because larger particulates in the air also scatter light more uniformly across wavelengths. The bluest, cleanest skies happen when the air is dry, clean, and contains mostly just the small gas molecules that produce Rayleigh scattering.
The sky is blue because of one of physics' most elegant phenomena: tiny air molecules scattering blue light more than red light. The same physics explains why sunsets are red, why distant mountains look bluish, and why hazy days look white. Lord Rayleigh's 19th-century equations still describe the daily color show overhead.
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