Why is the sunset so red?
When the sun is low, its light crosses many times more air to reach you than at noon. Air scatters blue light out of the beam along the way, so mostly orange and red are left. Dust, smoke and high clouds catching that light make it more dramatic.
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Why the sky is blue in the first place
Sunlight contains every color. Air molecules are tiny and scatter short-wavelength blue light much more strongly than long-wavelength red light, so blue gets bounced all over the sky and comes at you from every direction. That's the blue sky.
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Why sunset turns it red
At noon sunlight takes a short, steep path through the atmosphere. At sunset it comes in sideways, through dozens of times more air. By the time it reaches you, most of the blue and green has been scattered out of the beam, and what's left is yellow, orange and red. Clouds high enough to still be in sunlight after the sun sets for you reflect that red light back down, which is why the best color often comes 10 to 20 minutes after sunset.
Why some sunsets are spectacular
- Smoke and dust add particles that scatter even more blue away and can make the sun look deep red or orange.
- High, thin clouds to the west act as a screen for the color. A totally clear sky gives a gentler glow; a solid overcast blocks it.
- Clean, dry air after a storm gives crisp, saturated colors.
“Red sky at night” has a grain of truth in places where weather moves west to east: a colorful sunset means the sky to the west, where tomorrow's weather is coming from, is clear enough to let the sun shine through.
Questions people ask
- Why is the sun red when there's wildfire smoke?
- Smoke particles scatter and absorb so much blue and green light that only red and orange make it through, even in the middle of the day.
- Why are sunsets more colorful in fall and winter?
- Often the air is cleaner and drier, and the sun sets at a lower angle in many places, so the colors last longer.