Why is the sky blue?

Physics of the blue sky

• Core explanation centers on Rayleigh scattering: air molecules are much smaller than visible wavelengths, so scattering strength scales sharply with decreasing wavelength (∝ 1/λ⁴), hitting blue/violet much harder than red.
• “Resonant frequency” is clarified: visible light is far below the main electronic resonances of N₂/O₂ (in the UV), so the simple λ⁻⁴ law is only an approximation far from resonance; closer to resonance the behavior changes.
• A side thread notes that modern understanding ties scattering to density fluctuations in the gas, but for an ideal gas this reduces to the Rayleigh formula.

Why not violet, green, or other colors?

• Violet light scatters even more than blue, but our cones are less sensitive to violet; blue-sensitive cones dominate the perception, so the sky looks blue.
• Explanation for “why no green sky at sunset”: as path length increases, blue and then green are both preferentially scattered out of the direct solar beam, leaving mostly red; the intermediate mixtures of spectra produce oranges/yellows and then muddy browns, not a clean green band.
• The rare “green flash” at sunset is mentioned as a distinct refractive effect, not Rayleigh scattering.

Air/sky color and clouds

• There is repeated debate over whether “the air is blue” is an acceptable simplified answer.
  • One camp: if a large volume of air under sunlight appears blue, then it is blue in that context, just like a blue butterfly.
  • Other camp: color strongly depends on direction and mechanism (scattering vs absorption), so “air is blue” is oversimplified.
• Liquid oxygen’s obvious blueness is cited as related but only a small contributor to sky color.
• Clouds are described as collections of droplets that scatter all visible wavelengths roughly equally; they appear white when lit by near‑white illumination (direct sun plus blue skylight), and colored at sunrise/sunset.

Biology, perception, and evolution

• Discussion covers how cone sensitivities shape perceived sky color, color blindness (especially anomalous trichromats), and animals with different cone sets (birds, dogs, possible human tetrachromats).
• Several comments emphasize that “color” is a construct of the visual system; different species would not all see the sky as the same color.

Writing style and pedagogy

• Many readers praise the article’s depth, visuals, and layout, and request more posts/RSS.
• A side debate covers use of emojis and reassurance: some find it patronizing, others argue it helps non‑expert readers finish and enjoy technical explanations.