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.
By the Numbers
Key Facts
- Mechanism
- Rayleigh scattering
- Shorter wavelengths
- Scatter more strongly
- Sunset
- Longer atmospheric path removes more blue from direct sunlight
How sunlight turns the sky blue
- White sunlight arrives
It contains every visible color, from violet at about 400 nanometers to red at about 700.
- Air molecules scatter it
Nitrogen and oxygen molecules are far smaller than light’s wavelength, so they scatter short wavelengths much more strongly.
- Blue fills the sky
Scattered blue light reaches your eyes from every direction, not just from the Sun.
- Sunset stretches the path
Low sunlight crosses far more air, so most blue is scattered out of the direct beam, leaving reds and oranges.
Earth’s sky vs. Mars’s sky
| Earth | Mars | |
|---|---|---|
| Daytime sky | Blue | Butterscotch to rusty |
| Sunset, near the Sun | Red and orange | Blue |
| What scatters the light | Gas molecules far smaller than light’s wavelength | Fine dust, which sends blue light forward close to the Sun’s direction |
NASA’s Curiosity rover recorded its first color Martian sunset on April 15, 2015, in Gale Crater.
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.
Myth vs. Fact
MythThe sky is blue because it reflects the ocean.
FactThe sky is blue over deserts too. The color comes from air scattering sunlight.
MythIf shorter wavelengths scatter more, the sky should be violet.
FactViolet does scatter more, but sunlight contains less of it, some is absorbed high in the atmosphere, and our eyes are less sensitive to it.
The Bottom Line
The sky is blue because air molecules scatter the short, blue wavelengths of sunlight far more than the long, red ones, so blue light reaches your eyes from every direction.