Why is the sky blue?
The sky is blue because the tiny bits of gas in the air reflect blue sunlight around much more than red light, so blue reaches our eyes from every part of the sky.
The sky is blue because sunlight is a mix of every color, and the air reflects blue light around far more than red light. That scattered blue comes at us from all over the sky, so the whole sky looks blue. Scientists call this Rayleigh scattering. That is the short answer. The longer answer takes a few more steps, so let's go slowly. Here is how white sunlight hides a rainbow, why the tiny bits of air are so fond of blue, why the sky is not purple instead, why clouds stay white, and why sunsets turn red and pink. We will even take a quick trip to Mars, where the sky plays the whole trick the other way around.
Sunlight hides a rainbow
Sunlight looks plain white, but it is really every color mixed together. NASA's Space Place, a science site for kids, points out that a prism can pull white light apart into all the colors of the rainbow. A prism is just a specially shaped piece of glass or crystal. Rain can do the same thing, which is why you see a rainbow when the sun comes out after a shower. So the colors were in the sunlight all along. They were simply traveling together. When they all reach your eyes at once, your brain reads the mix as white. The sky's blue is what happens when part of that mix gets pulled away from the rest and sent off in a new direction.
Light travels in waves, a bit like ripples on a pond. The distance from the top of one wave to the top of the next is called the wavelength. Each color has its own wavelength. NASA explains that red light moves in long, lazy waves, while blue light moves in short, choppy ones. A physics guide first written by Philip Gibbs for a university physics FAQ gives the numbers. Red light has a wavelength of about 720 nanometers and violet about 380. A nanometer is one billionth of a meter, so these waves are far too small to see. Still, that difference in size between red and blue is the whole secret behind the color of the sky.
Air loves to reflect blue light
The air around you is made of tiny particles called molecules, far too small to see. NASA says Earth's air is about 78 percent nitrogen and 21 percent oxygen, with small amounts of other gases. When sunlight hits these molecules, some of the light gets reflected off in a new direction. That reflecting is called scattering. The Georgia State University physics site HyperPhysics explains that this kind of scattering off air molecules is called Rayleigh scattering, after Lord Rayleigh, a British scientist who studied it in the 1800s. The key point is that it works much better on short waves than on long ones. So the air grabs blue light and sends it flying all over the place, while red light mostly passes straight through.
How much better is blue at getting reflected? Lord Rayleigh worked out a neat rule. Halve the wavelength and the scattering gets sixteen times stronger. That sounds like a lot, and it is. Using this rule, the university physics FAQ works out that blue light is scattered about ten times more than red light. That scattered blue light zigzags across the sky and comes at your eyes from every direction, not just from where the sun is. So when you look up at any patch of clear sky, you are seeing sunlight that has been reflected your way by air. NASA adds that near the horizon the sky fades to a paler blue or white, because that light has been reflected so many times the colors mix again.
The violet puzzle
Here is a clever question. Violet has an even shorter wavelength than blue. So violet should get reflected around even more. Why isn't the sky violet? The university physics FAQ gives a three part answer. First, the sun does not give off the same amount of every color, and it sends us less violet than blue. Second, some of the violet is soaked up high in the atmosphere before it gets down to us. Third, our eyes are simply not very sensitive to violet. The encyclopedia Britannica agrees on that last point, saying our eyes are more sensitive to blue light than to violet light. Put together, there is less violet to start with, and we are not great at noticing it.
There is one more piece, and it is about how our eyes work. The back of your eye has three kinds of color sensors, called cones. People often call them red, green and blue cones, because each one reacts most to light near that color. Your brain works out colors by comparing how strongly each type is buzzing. According to the physics FAQ, sky light sets off the red and green cones by about the same amount, and the blue cones much more strongly. Your brain reads that particular mix as a soft, pale blue. So the sky does have some violet in it. It just blends into the blue we see, like a pinch of salt you cannot taste on its own.
Why clouds stay white
If air turns sunlight blue, why are clouds white? It comes down to size. Clouds are made of tiny drops of water, and HyperPhysics notes that these drops are much bigger than the molecules of the air. That changes the game. Tiny air molecules pick favorites and reflect blue the most. Bigger drops, bigger than the waves of light themselves, reflect all the colors about equally. Scientists call this Mie scattering, after the German physicist Gustav Mie. When every color gets reflected the same amount, the mix that reaches your eyes is still white. So a cloud is basically a floating pile of tiny water drops, all tossing back plain sunlight with no favorites at all.
The same idea explains other white things in the air. HyperPhysics says Mie scattering gives us the white light from mist and fog, and the almost white glare around the sun on a hazy day. The university physics FAQ adds that dust haze looks white for the same reason. HyperPhysics also points out something you can check yourself on a clear day. The blue of the sky is deepest when you look far away from the sun, and it gets paler and whiter as you look closer to it. That is because more of the light near the sun comes from bigger particles, which reflect every color the same. Next time you see a fluffy white cloud against a deep blue sky, you are really looking at two kinds of scattering side by side.
Red and pink skies at sunset
At midday, sunlight comes down from high overhead and takes a fairly short trip through the air. At sunrise and sunset the sun sits low, so its light has to slide sideways through a much thicker slice of air to reach you. NASA's Space Place explains that along that long path, even more of the blue light gets scattered away, which lets the redder light pass straight through to your eyes. Think of it like a long line of people each taking a little blue out of the sunlight as it goes past. By the end of the line, most of the blue is gone. What is left over is the warm, reddish light that paints the clouds at the end of the day.
Some evenings are much redder than others. NASA says small particles of dust, pollution or other bits in the air also scatter blue light, leaving even more red to come through. The physics FAQ agrees that a sunset gets redder when the air holds more small particles. NASA also notes that extra colorful sunrises and sunsets sometimes happen after powerful volcanoes throw tiny dust grains high into the sky. After the sun dips below the horizon, the show often keeps going. In a 2003 study of twilight, scientist Raymond Lee wrote that during many clear twilights much of the sky near the sun turns pastel purple. So if you keep watching after sunset, you may get red, then rose and pink, then a soft purple glow.
The sky on Mars
Other planets have other skies, and Mars is the famous one. According to NASA's Space Place, the air on Mars is thin, mostly carbon dioxide, and full of fine dust. During the day, that dust gives the Martian sky a reddish color. Then comes the twist. At sunset, the sky around the sun turns a cool blue gray, which is the opposite of what happens on Earth. NASA's Curiosity rover photographed one of these blue sunsets in Gale Crater on April 15, 2015. Picture standing on Mars under a dusty red sky all afternoon, then watching the sun slip away inside a soft blue glow. It would feel like the whole sky had been flipped inside out.
Why the swap? It is all about the dust. Mark Lemmon of Texas A&M University, a scientist on the Curiosity team, told NASA that the very fine dust is just the right size to let blue light pass through the air slightly more easily. When blue light reflects off that dust, it tends to stay close to the direction of the sun, so the blue gathers around the setting sun. NASA adds that Martian twilight lasts much longer than ours, with a faint glow for up to two hours before sunrise or after sunset. That is because dust high in the sky scatters sunlight around to the night side of the planet. Same sunlight, different air, completely different sunset.
So the next time someone asks why the sky is blue, you have the answer. Sunlight carries every color. The tiny molecules in the air reflect blue around the most, so blue reaches us from every corner of the sky. Violet gets reflected too, but there is less of it and our eyes barely notice. Clouds stay white because their water drops reflect every color the same, and sunsets go red and pink because the light takes the long way through the air. Here is a little challenge for this week. Pick a clear evening, step outside when the sun starts to set, and watch the colors change from blue to red, then rose, then purple. You will know exactly what the sky is up to.
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