Sooner or later, every curious kid looks up and asks it: why is the sky blue? It's tempting to guess that the sky simply reflects the ocean, but that's not the reason. The real answer is much better, and once you know it, you'll never look at a sunset the same way.
Here's the short version. Sunlight looks white, but it's actually a mix of all the colors of the rainbow. As it passes through the air, tiny gas molecules scatter it in every direction. Blue light, with its shorter wavelengths, gets scattered much more strongly than red or yellow light. So when you look at any part of the sky away from the sun, you're seeing that scattered blue light coming at you from all over.
Meet Rayleigh Scattering
Scientists call this effect Rayleigh scattering, after the 19th-century British physicist Lord Rayleigh. It happens when light bounces off particles much smaller than the light's wavelength, like the nitrogen and oxygen molecules that make up most of our air.
The key detail is how strongly the effect depends on color. The amount of scattering grows dramatically as the wavelength gets shorter. Blue is scattered much more than red, which is why blue dominates the daytime sky.
What about violet, which has an even shorter wavelength? The scattered light from the sky is really a blend of blue and violet. Our eyes respond to that mix as if it were blue mixed with white light, which is why the sky looks like a soft, pale blue rather than a deep purple.
Why Sunsets Glow Orange and Red
At sunrise and sunset, the sun sits low on the horizon. Its light has to travel through a much longer stretch of atmosphere to reach your eyes. Along the way, much of the blue and green light gets scattered out of the beam.
What's left are the warmer colors. That's why the low sun, and the clouds it lights up, can glow in brilliant shades of orange and red. Interestingly, near sunset and during twilight, ozone high in the atmosphere helps keep the evening sky overhead blue.
And Why Clouds Are White
If air scatters blue light, why aren't clouds blue too? It comes down to size. Clouds are made of water droplets that are about the same size as the wavelengths of visible light. Those droplets scatter all colors roughly equally, so clouds look white or gray instead.
Centuries of Curious Minds
People have wondered about the sky's color for a very long time. Around 1506 to 1510, Leonardo da Vinci suggested that tiny particles in the air scattering sunlight gave the sky its blue hue.
In 1869, the scientist John Tyndall noticed that bright light scattering off tiny particles had a faint blue tint. He thought it might explain the sky, but he couldn't explain why blue was favored. In 1871, Lord Rayleigh published papers working out the math, and by 1899 he had shown that the effect applies to individual molecules of air. That work became the basic scientific model for the color of the sky.
So the next time a child asks you why the sky is blue, you'll have a great answer ready. Sunlight is a rainbow in disguise, and our air is constantly tossing the blue part of it around, painting the whole sky above us.
