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.
Quick Answer
Sunlight looks white because it contains many visible wavelengths. When it enters Earth’s atmosphere, molecules scatter the shorter wavelengths much more strongly than the longer red wavelengths. Blue light is therefore scattered across the sky and reaches your eyes from many directions. Violet is even shorter, but the Sun emits less of it, some is absorbed, and human vision is less sensitive to violet, so the sky appears blue rather than violet. At sunrise and sunset, sunlight travels through more atmosphere, scattering much of the blue light out of the direct path and leaving warmer reds and oranges.
Key Facts
- Mechanism
- Rayleigh scattering
- Shorter wavelengths
- Scatter more strongly
- Sunset
- 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.