It is one of the first science questions many children ask: why is the sky blue? The answer is both simple and profound. It involves the nature of sunlight, the size of the molecules in the air and the way our eyes see color. The same physics explains why sunsets glow orange and red, why clouds are white, why distant mountains look bluish and why the sky on Mars has a very different color. This explainer walks through the science step by step.
Sunlight contains all colors
Sunlight looks white or slightly yellowish, but it is actually a mixture of all the colors of the rainbow. You can see this when sunlight passes through a prism or raindrops and spreads out into a spectrum of red, orange, yellow, green, blue, indigo and violet.
Light is an electromagnetic wave, and each color corresponds to a different wavelength:
| Color | Approximate wavelength |
|---|---|
| Red | 620 to 750 nanometers |
| Orange | 590 to 620 nanometers |
| Yellow | 570 to 590 nanometers |
| Green | 495 to 570 nanometers |
| Blue | 450 to 495 nanometers |
| Violet | 380 to 450 nanometers |
A nanometer is a billionth of a meter. Blue and violet light have shorter wavelengths, while red light has longer wavelengths.
Light meets the atmosphere
Earth's atmosphere is made mostly of nitrogen and oxygen molecules, along with small amounts of other gases, water vapor and tiny particles. When sunlight travels through the atmosphere, it interacts with these molecules.
Air molecules are much smaller than the wavelength of visible light. When light waves pass by these tiny molecules, the electric field of the light causes the electrons in the molecules to oscillate. The oscillating electrons then radiate light in all directions. This process is called scattering.
Rayleigh scattering: short wavelengths scatter more
The key to the blue sky is that scattering by molecules much smaller than the wavelength of light depends strongly on wavelength. This type of scattering is called Rayleigh scattering, after the British physicist Lord Rayleigh, who explained it in the 1870s.
Rayleigh found that the amount of scattering is inversely proportional to the fourth power of the wavelength. In simple terms, shorter wavelengths are scattered much more strongly than longer ones. Blue light, with a wavelength of about 450 nanometers, is scattered more than five times as strongly as red light, with a wavelength of about 700 nanometers.
As sunlight passes through the atmosphere, blue light is scattered in all directions by air molecules, filling the sky. When you look at any part of the sky away from the Sun, you see this scattered blue light coming toward you from all directions. That is why the sky looks blue.
Why is the sky not violet?
If shorter wavelengths scatter more, and violet light has an even shorter wavelength than blue, why is the sky not violet? There are three reasons:
- Sunlight contains less violet than blue. The Sun's light is not equally bright at all wavelengths, and some violet and ultraviolet light is absorbed high in the atmosphere.
- Our eyes are less sensitive to violet. Human color vision depends on three types of cone cells in the retina. They respond strongly to blue, green and red light but are relatively insensitive to violet.
- The mixture of scattered colors looks blue. The scattered light contains some violet, blue, green and a little of the other colors. Our brain interprets this combination as a pale, sky blue.
Why sunsets are red and orange
At sunrise and sunset, the Sun is low on the horizon. Its light must travel through much more of the atmosphere to reach your eyes, sometimes dozens of times more than when the Sun is overhead.
Along this long path, most of the blue and violet light is scattered out of the direct beam before it reaches you. The light that remains is dominated by longer wavelengths: yellow, orange and red. That is why the Sun and the sky near it turn warm colors at sunrise and sunset.
Dust, smoke, pollution and volcanic particles in the atmosphere can make sunsets even more vivid, by scattering and absorbing additional light. After major volcanic eruptions, people around the world have reported unusually colorful sunsets for months.
Why the sky is pale near the horizon
The sky directly overhead often looks deeper blue than the sky near the horizon. Near the horizon, you are looking through more atmosphere. The light reaching you has been scattered many times, mixing colors together and making the sky appear lighter, closer to white.
Why clouds are white
Clouds are made of water droplets and ice crystals that are much larger than air molecules, often about the same size as or larger than the wavelength of visible light. Particles of this size scatter all wavelengths roughly equally, a process sometimes called Mie scattering. Because they scatter all colors equally, clouds look white. Thick clouds look gray because less light passes through them.
Haze, fog and pollution particles also scatter light of all colors, which is why polluted or humid skies often look whitish rather than deep blue.
Why distant mountains look blue
Distant mountains and hills often appear bluish and faint. This is because the air between you and the mountains scatters blue light toward your eyes, adding a blue haze over the view. Painters use this effect, called aerial or atmospheric perspective, to suggest distance in landscapes.
The sky from space and on other worlds
Without an atmosphere, there is nothing to scatter sunlight, so the sky looks black even in daylight. Astronauts on the Moon saw a black sky with the bright Sun and Earth visible at the same time.
On Mars, the atmosphere is very thin and contains a lot of fine reddish dust. During the day, the dust scatters light in a way that makes the sky look butterscotch or pinkish. Surprisingly, Martian sunsets look bluish, because the fine dust particles scatter blue light forward, around the setting Sun.
On other planets, the color of the sky depends on the composition of the atmosphere and the particles in it.
Polarized skylight
Scattered sunlight is also partly polarized, meaning its waves tend to vibrate in a particular direction. The polarization is strongest about 90 degrees away from the Sun. Polarized sunglasses can make the sky look darker blue in certain directions. Some animals, such as bees, can detect polarized light and use it to navigate.
Try it yourself
You can create a miniature blue sky and sunset at home. Fill a clear glass with water and add a few drops of milk, stirring gently. Shine a flashlight through the side of the glass in a dark room. Looking from the side, the water will appear slightly bluish, because the tiny milk particles scatter blue light. Looking through the glass toward the flashlight, the light will appear yellowish or orange, like a sunset, because blue light has been scattered away from the direct beam.
Other colors in the sky
Scattering, reflection and refraction create many other sky colors and effects:
- Rainbows form when sunlight enters raindrops, bends, reflects off the back of the drop and bends again as it leaves. Different colors bend by slightly different amounts, spreading the light into a spectrum. You see a rainbow when the Sun is behind you and rain is in front of you, at an angle of about 42 degrees from the point directly opposite the Sun.
- Twilight colors linger after sunset because sunlight still illuminates the upper atmosphere. The deep blue of the sky shortly after sunset comes partly from ozone high in the atmosphere, which absorbs some orange and red light.
- Halos around the Sun or Moon appear when light passes through tiny ice crystals in high, thin clouds. The crystals bend light at an angle of about 22 degrees, creating a ring.
- The Belt of Venus is a pinkish band sometimes seen above the horizon opposite the setting Sun, just above the bluish shadow of Earth cast on the atmosphere.
- Green flashes are brief flashes of green light occasionally seen just as the Sun sets or rises over a clear, distant horizon, caused by the atmosphere bending different colors by different amounts.
A short history of the question
People have wondered about the color of the sky for thousands of years. Leonardo da Vinci suggested in the early 1500s that the sky's blue came from light interacting with particles in the air. In the 19th century, the Irish physicist John Tyndall showed in experiments that small particles scatter blue light more than red, an effect still called the Tyndall effect for scattering by small particles in liquids and gases. Lord Rayleigh then worked out the mathematics, showing that even the gas molecules of the air are enough to make the sky blue. Later, Albert Einstein refined the theory, connecting the scattering of light to tiny fluctuations in the density of air.
Questions readers ask
Why is the sky blue? Air molecules scatter short-wavelength blue light much more than long-wavelength red light. This scattered blue light reaches our eyes from all directions in the sky.
Why are sunsets red? At sunset, sunlight travels through more atmosphere, and most of the blue light is scattered away, leaving mostly red and orange light.
Why is the sky not violet? Sunlight contains less violet light, some is absorbed high in the atmosphere, and our eyes are less sensitive to violet.
Why are clouds white? Cloud droplets are large enough to scatter all colors of light roughly equally, making them appear white.
What color is the sky on Mars? During the day it looks butterscotch or pinkish because of dust, and at sunset it can look bluish.
In short
The sky is blue because sunlight, a mixture of all colors, is scattered by the tiny molecules of air, and short-wavelength blue light is scattered much more strongly than red light. This scattered blue light reaches us from every direction. At sunrise and sunset, sunlight travels through more air, so blue is scattered away and warm colors dominate. Larger particles in clouds and haze scatter all colors equally, making them white, while the absence of air makes the sky black in space. Next time you look up, you will be seeing the physics of light painted across the sky.