Why is the sky blue?
Air molecules scatter shorter wavelengths of visible sunlight more strongly than longer wavelengths, sending blue light across the sky toward your eyes.
Key points
Rayleigh scattering
Scatter more strongly
Longer atmospheric path removes more blue from direct sunlight
The fourth-power effect
For particles much smaller than the wavelength of light, Rayleigh scattering becomes dramatically stronger as wavelength gets shorter. That makes blue light far more likely than red light to be redirected by air molecules.
Why not violet
Physics alone would suggest strong violet scattering too. But the color we perceive also depends on the spectrum of sunlight, atmospheric absorption and the sensitivity of the human eye.
Why clouds are white
Cloud droplets are much larger than individual air molecules and scatter visible wavelengths more evenly. That is why clouds tend to look white or gray rather than blue.
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