Why is the sky orange at sunrise and sunset?
Key takeaways
- Sunlight is a mix of all colors. Air molecules scatter short blue waves far more than long red ones, a process called Rayleigh scattering.
- At sunrise and sunset, sunlight crosses much more atmosphere, so the blue scatters away and orange and red light reaches your eyes.
- Dust, smoke, and volcanic particles deepen the colors. For vivid sunsets, watch with clean air and a few high, thin clouds to the west.
The orange glow of sunrise and sunset is sunlight filtered by air. When the Sun sits low, its light takes a long, slanted trip through the atmosphere, and that trip strips away the blue.
What is left is the warm end of sunlight: yellow, orange, and red. The Sun itself has not changed color at all. The air between you and it has.
This article explains why the sky is orange during sunrise and sunset. It covers what makes the sky change color, why orange and red appear at sunrise and sunset, why the sky looks different at noon, what changes the colors, and the bottom line.
The sky glows orange as the Sun sets over Li Phi falls in Laos. Photograph: Basile Morin/Wikimedia Commons
Image Credits: Wikimedia Commons
What makes the sky change color?
Sunlight looks white, but it is really a mix of all the colors of the rainbow, according to NOAA. Each color travels as a wave, and the waves come in different lengths. The red waves are the longest we can see. Blue and violet are the shortest.
When sunlight enters the atmosphere, it runs into gas molecules, mostly nitrogen and oxygen. These molecules are far smaller than the wavelengths of visible light. They scatter the light in all directions, and they scatter short waves much more strongly than long ones.
NASA Space Place puts it simply: blue light is scattered more than the other colors because it travels as shorter, smaller waves. That scattered blue reaches your eyes from every part of the sky. That is the whole trick behind a blue sky.
Scientists call this wavelength-selective scattering Rayleigh scattering, after the British physicist Lord Rayleigh, who worked out the mathematics in the 1870s. The core idea is easy to hold onto: the shorter the wave, the harder it bounces off air molecules.
Why orange and red at sunrise and sunset
At sunrise and sunset, the Sun sits near the horizon. Its light no longer drops straight down through the air. Instead it travels sideways through a much thicker slice of atmosphere to reach your eyes, as the UCAR Center for Science Education explains.
All that extra air means extra scattering. The blue light that painted the midday sky gets scattered away long before the beam reaches you. What survives the trip is the long-wave end of sunlight.
"Most of the blue has been scattered out of that beam," Stephen Corfidi, a meteorologist with NOAA, told Scientific American. What remains, he said, are the warmer hues of yellow, orange, and red blending into the sunset glow.
NASA Space Place describes the same handoff: as the Sun gets lower, even more blue light is scattered, allowing the reds and yellows to pass straight through to your eyes. That is why the Sun itself looks orange near the horizon, and why clouds lit from below catch the same warm tones.
Rayleigh scattering: air molecules scatter short blue wavelengths in all directions while longer red wavelengths pass through. Diagram: Robert A. Rohde/Wikimedia Commons
Image Credits: Wikimedia Commons
Why the sky looks different at noon
At midday the geometry flips. The Sun is nearly overhead, so its light takes the shortest possible path down, passing through a relatively thinner section of the atmosphere.
With less air in the way, less blue gets scattered out of the beam. Plenty of blue still scatters in every direction, and because it arrives from the whole sky at once, the sky overhead reads as blue. The direct sunlight reaching the ground stays close to white.
So the noon sky and the sunset sky are the same physics at two path lengths. Short trip through the air, and scattered blue dominates. Long trip, and the blue is gone before the light arrives.
Other planets run the same physics with different air. NASA notes that Mars has a thin atmosphere of mostly carbon dioxide filled with fine dust. The result is the mirror image of Earth: an orange daytime sky with sunsets that glow blue-gray around the Sun, captured in images from NASA's Mars Pathfinder lander.
What changes the colors
Clean air gives soft golden sunsets. Particles in the air change the show. NASA Space Place notes that small particles of dust, pollution, or other aerosols also scatter blue light, leaving more purely red and yellow light to pass through.
When the air holds lots of aerosols, even more sunlight is scattered and absorbed, and the UCAR Center for Science Education says this makes sunsets more colorful and red. Desert dust, sea spray, and wildfire smoke all add particles that strip away the short waves.
Human-made haze joins in. "It is certainly true that the 'pollution' results in redder sunsets," Craig Bohren, professor emeritus of meteorology at Pennsylvania State University, told Scientific American. Scattering by nitrogen and oxygen alone, he said, can only push a sunset toward orange, not blood red.
Volcanoes supply the most dramatic particles of all. A. R. Ravishankara, director of chemical sciences at NOAA's Earth System Research Laboratory in Boulder, Colorado, explained that eruptions can inject sulfuric acid droplets into the stratosphere, 10 to 35 miles up.
Those droplets circle the globe for months, painting crimson twilights far from the eruption. After Indonesia's Krakatoa erupted in 1883, brilliant sunsets appeared around the world, and one is said to have inspired Edvard Munch's painting The Scream.
Clouds add the final layer. High, thin clouds catch the low Sun's warm light and throw pink, orange, and purple across a wide sky. Low, thick clouds do the opposite and block the light before it can perform.
Warm light spreads across the sky at sunrise over Sugarloaf Mountain in Rio de Janeiro. Photograph: Donatas Dabravolskas/Wikimedia Commons
Image Credits: Wikimedia Commons
Frequently asked questions
Why is the sky orange at sunset but blue at noon?
It is the same scattering at two different path lengths. At noon sunlight crosses little air, so scattered blue fills the sky. At sunset it crosses much more air, the blue scatters away, and orange and red remain.
Do smoke and pollution make sunsets more colorful?
They can. Extra particles scatter away more blue light, which deepens the reds and oranges. But heavy low haze can also wash the sky pale and gray, so more particles do not always mean a better sunset.
Why are some sunsets pink or purple instead of orange?
The exact color depends on how much of each wavelength survives the trip and what the light strikes. High, thin clouds can catch the warm light and spread pink and purple tones across a wide sky.
Does the same thing happen on other planets?
Same physics, different air, different colors. Images from NASA's Mars Pathfinder lander show an orange daytime sky on Mars and sunsets with a blue-gray glow around the Sun, the reverse of Earth, because fine dust scatters light differently there.
The bottom line
The orange sky at sunrise and sunset is blue light lost along the way. Sunlight takes a long, low path through the atmosphere, air molecules scatter the short blue waves out of the beam, and the long orange and red waves arrive at your eyes. Dust, smoke, and volcanic particles deepen the effect by scattering away even more blue.
For the most vivid sunsets, watch on days with clean air and a few high, thin clouds in the west to catch the warm light, and give the Sun a clear horizon to sink behind.
Rive News's research desk publishes plain-English explainers on science, technology, health, and money, built from primary sources and checked against the lab's QA gate.
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