Why Is The Sky Blue?

Gazing into the deep blue sky can invoke a sense that you're peering into the infinite. That sea of blue you see above your head looks like it never ends, but it may shock you that it only appears blue to us due to a certain physical process. While it unfortunately doesn't involve any mysterious green ghosts in the sky, it does involve a fair amount of science.

The sky is blue thanks to Rayleigh scattering: a physical phenomenon where sunlight becomes scattered due to interacting with molecules and debris in the Earth's atmosphere, leaving a blue color to be the predominant visible wavelength. Along with being responsible for the color of the sky, Rayleigh scattering is also one of the reasons an individual's eyes can appear green.

This process is also responsible for how a sunrise or sunset may appear, as it helps completely scatter blue wavelengths to produce those familiar red and orange hues. It also provides our occasionally photogenic sun its familiar yellow color when looking at it from Earth. When Rayleigh scattering joins forces with another scientific phenomenon, the results can be completely jaw dropping.

Before jumping into what makes the sky blue, here's a quick primer. The sunlight that enters the Earth's atmosphere emits nearly every wavelength on the electromagnetic spectrum, though not in equal parts. This spectrum contains all types of electromagnetic radiation, including microwaves, X-rays, infrared light, ultraviolet light, and visible light. Visible light is what our eyes perceive as colors, and all of them are associated with specific wavelengths on the spectrum. Blues and purples have shorter wavelengths, whereas reds and oranges have longer wavelengths.

As sunlight enters the atmosphere of the Earth, it interacts with the molecules present within it, including oxygen, nitrogen, and other minute gases and debris. Since these molecules are smaller than light waves, they scatter the wavelengths, and shorter waves like blue are scattered more strongly than longer waves like red. This process is known as the previously mentioned Rayleigh scattering.

Essentially, the light from the sun interacts with molecules in our atmosphere, which distributes wavelengths to the blue end of the visible spectrum. This creates more blue light wavelengths over any other, giving the sky its familiar appearance. As famed scientist Bill Nye puts it, "The sky is blue because the molecules in the Earth's air are just the right size and just the right distance apart to cause the blue light to scatter more than the red light" (via Sophia Learning). Due to the cones that detect light in our eyes, it's difficult for us to see the hues of purple that should also be present in the sky.

As the sun becomes lower in the sky, its light needs to pass through a thicker part of the Earth's atmosphere. The thickness of the atmosphere combined with a longer traveling distance scatters more blue light further, leaving only light in the red and orange parts of the spectrum. "At sunset, so much blue light is scattered so strongly, because the atmosphere is a little thicker with respect to the path of the sun, that all that's left is red light," says Nye. "It's not magic. It's science."

Along with atmospheric molecules having an impact on the color of the sky, cloud droplets can also influence how the sky may look. If the droplets are equal in size or larger than the wavelengths, then the visible wavelengths will scatter across the sky equally. This process is known as Mie scattering, and it's why a cloud may appear white. Thicker clouds may also appear gray due to the blocking of more light, or the cloud may even have a slightly blue color if it is not receiving any direct sunlight and it reflects the color of the sky.

If you've ever seen a gorgeous sunrise where the sun overtakes the clouds, it's because of Rayleigh scattering and Mie scattering working in tandem. Rayleigh scattering helps eliminate all the blues, while Mie scattering emphasizes the reds in the clouds, creating a visual image that can be absolutely stunning to enjoy.