There’s something distinctly uplifting about a blue sky on a sunny day. But many of us have become so accustomed to this cheerful shade of light blue that we don’t always notice it. And if you’re like many people, you may not have ever stopped to wonder what makes the sky blue.
Here’s a look at where that all-too-familiar blue color comes from and how the sky’s color changes.
The Many Colors of Sunlight
The sun’s light is made up of electromagnetic radiation, meaning it’s a combination of vibrating electric and magnetic fields. Electromagnetic radiation comes in the form of waves, and some of these waves contain more energy than others.
Higher energy forms of light have shorter wavelengths (distance between the peak of each wave) and a higher frequency (number of waves per second). Here’s an illustration of the spectrum of sunlight:
To the right of the spectrum, we see infrared light. The wave pattern has a long wavelength and low frequency, so this is lower-energy light. It’s outside of the visible spectrum, so we can’t see infrared light.
But as the wavelength shrinks and the frequency increases, the light becomes visible (as red light). As we move through the colors of the rainbow, the wavelengths continue to shrink, and the frequency continues to increase. That means that blue light and violet light are higher-energy wavelengths than red light.
After violet light comes ultraviolet (UV) radiation. This ultra-high-energy light is outside of the visible spectrum of light for humans. However, some animals — including reindeer, butterflies, birds, and reptiles — can see UV light.
With all of these colors in the visible spectrum, you might wonder why the sun’s light appears white. That’s because when all of these colors are radiated together, they look white. But when they are refracted (like through a prism) and split into their respective wavelengths, you can see each color individually.
What Makes the Sky Blue?
The sun’s white light travels through space at the rate of 186,282 miles/sec until it reaches Earth’s atmosphere. That’s where it begins to split into different colors. But before we look into how that happens, we need to understand how light behaves in the air.
White light travels in a straight line until it bumps into something. That “something” can be as small as a water droplet or even a single molecule.
Bits of dust and water molecules are large enough that they bounce white light off in different directions. But when white light runs into a tiny gas molecule, it’s a different story altogether.
Gas molecules are smaller than a wavelength of light. If white light hits a gas molecule, all of the colors within it can be absorbed. But higher-energy wavelengths (like blue and violet) are absorbed more often.
When gas molecules absorb these colors, they don’t keep them forever. Instead, they start to radiate them in different directions. These wavelengths then go on to be absorbed and radiated countless times.
This phenomenon is called Rayleigh scattering. It’s named for English physicist Lord John Rayleigh, who described the process in 1871. In a paper called “On the Light from the Sky, Its Polarization and Colour,” he determined that blue light, with its shorter wavelength, was scattered 16 times more than red light. Because blue light was scattered so frequently, the sky appeared (and continues to appear) blue.
You may have noted above that high-energy wavelengths of light are scattered more frequently. And if you look at the diagram of the visible spectrum of light, you’ll see that violet light has more energy than blue light. So why isn’t the sky purple?
There are two main reasons why. First, the sun produces more blue light than violet light. Our eyes are also much more sensitive to blue than they are to purple.
Human eyes include two main types of cells for detecting light, shadow, and color. Rods help us see in dim light. Cones detect color. We have three different types of cones: one for low-frequency light (red), one for medium-frequency light (green), and one for high-frequency light (blue).
Violet is a high-frequency wavelength that would primarily be detected by the so-called “blue” cone. However, this cone’s peak sensitivity is in the range of light we typically see as blue. While it can pick up violet light, it’s much less sensitive to these higher frequencies.
Why Isn’t the Sky Blue All the Time?
During daylight hours, the sky is usually blue unless it’s obscured by clouds. But as you’ve seen, sunrises and sunsets are often full of impossibly bright colors like red, orange, pink, and yellow. Where do those colors come from? And what happens to blue light?
This color change has to do with how far sunlight needs to travel to reach the Earth. When the sun is low on the horizon, its light has to travel through more of the Earth’s atmosphere before it gets to us. That means that the light runs into more gas molecules that scatter it.
The blue light is still being scattered — it just scatters off long before it reaches our field of vision. That means that the longer wavelengths of light (red, orange, and yellow) reach us in greater numbers. As a result, the sky looks bright and fiery until the sunrise or sunset is completed.
Particles in the Atmosphere
Unfortunately, some of these ultra-brilliant sunrises and sunsets have been influenced by air pollution. When air is polluted, there are a greater number of particles suspended in the atmosphere. These particles scatter light just like natural gas molecules do. With more blue light scattered off early on, the warm colors look even more vivid.
You may have heard of (or even seen) the spectacular sunsets in Southern California. The unfortunate truth is that air pollution contributes to these vibrant shades by increasing the sky’s overall brightness.
However, you don’t have to wait for a sunrise or sunset to see the effects of air pollution on the color of the sky. In very polluted cities like Los Angeles or Shanghai, there’s often a hazy, yellowish “cloud” floating above. It’s especially visible from a distance if you look along the city skyline.
This type of air pollution is colloquially referred to as “smog.” It tends to look yellow or orangish for the same reason sunsets do — because it introduces so many extra particles to the atmosphere, it scatters off more blue light, letting only longer wavelengths shine through.
Will the Sky Always Be Blue?
You might take the blue sky as a given. But if air pollution around the world continues at its current rate, it’s possible that the sky could change color from its current vibrant blue to a sickly, yellowish shade.
As pollution grows, there may be enough extra particles in the air to scatter away even yellow and orange wavelengths of light, creating an ominous red. It’s also possible that air pollution will increase so much that most wavelengths of light will be scattered away, leaving a dull gray sky.
Of course, if this change were to occur, it wouldn’t happen overnight. There’s still time for us to take action to reduce air pollution and preserve our planet’s beautiful blue sky.