Why is the sky blue?
Sunlight holds every color. The tiny molecules in air scatter short, blue wavelengths far more than long red ones, about six times more, so blue light bounces around the whole sky and reaches your eyes from every direction.
A deep blue sky means strong sun: UV is high.
A milky sky means particles in the air you breathe.
Show me why1 min
Scattering, in one picture
Nitrogen and oxygen molecules are much smaller than a wavelength of light. Particles that small scatter light far more strongly the shorter its wavelength, an effect called Rayleigh scattering. Blue light (about 450 nanometers) gets scattered roughly six times more than red (about 700 nanometers). Look anywhere away from the sun and you're seeing sunlight that was bounced toward you, mostly blue.
Go deeper2 more
Why not violet?
Violet is scattered even more than blue, but there's less of it in sunlight to begin with, some is absorbed high in the atmosphere, and our eyes are much less sensitive to it. The mix our eyes receive adds up to sky blue.
Why the sky changes color
- Paler near the horizon: light from there crosses much more air, gets scattered over and over, and arrives washed out.
- Red at sunset: low sunlight crosses so much air that the blue is scattered out before it reaches you, leaving reds and oranges.
- White or milky: bigger particles like haze, smoke, dust and water droplets scatter all colors about equally, so the sky goes white or gray. A deep blue sky means clean, dry air.
- Darker blue up high: on a mountain or from a plane there's less air above you to scatter light.
Questions people ask
- Why is the sky a deeper blue on some days?
- Clean, dry air has few large particles to whiten it. After a cold front or a good rain the sky often looks its bluest.
- Why are clouds white if the sky is blue?
- Cloud droplets are far bigger than air molecules and scatter all colors about equally, which adds up to white. Thick clouds look gray because less light gets through.
- What color is the sky on Mars?
- Butterscotch by day, because fine dust dominates the scattering there, and its sunsets look bluish.