**Why Is The Sky Blue?**
Looking up at the vast blue sky can create a feeling of gazing into the endless. That expanse of blue hovering above seems limitless, yet it may surprise you to learn that it only appears blue to us because of a specific physical process. Although it sadly doesn’t involve any enigmatic green spirits in the sky, it does encompass a considerable amount of science.
The blue hue of the sky is due to Rayleigh scattering: a physical occurrence where sunlight is scattered as it interacts with molecules and particles in the Earth’s atmosphere, resulting in blue being the main visible wavelength. In addition to determining the color of the sky, Rayleigh scattering is also a reason why some people’s eyes can appear green.
This phenomenon also influences the appearance of sunrises and sunsets, as it contributes to the scattering of blue wavelengths, producing those recognizable red and orange tones. It also gives our occasionally picturesque sun its characteristic yellow color when viewed from Earth. When Rayleigh scattering collaborates with another scientific phenomenon, the outcomes can be truly breathtaking.
**What is the real color of the sky?**
Before diving into the reasons behind the blue sky, here’s a brief introduction. The sunlight that penetrates the Earth’s atmosphere radiates nearly every wavelength in the electromagnetic spectrum, albeit not in equal amounts. This spectrum embodies all varieties of electromagnetic radiation, including microwaves, X-rays, infrared light, ultraviolet light, and visible light. Visible light represents the colors our eyes interpret, all linked to specific wavelengths on the spectrum. Blues and purples have shorter wavelengths, while reds and oranges possess longer wavelengths.
As sunlight passes into the Earth’s atmosphere, it interacts with the molecules within it, such as oxygen, nitrogen, and other tiny gases and particles. Since these molecules are smaller than light waves, they scatter the wavelengths, with shorter waves like blue scattering more than longer waves like red. This process is what we previously referred to as Rayleigh scattering.
In essence, sunlight interacts with atmospheric molecules, redistributing wavelengths towards the blue end of the visible spectrum. This results in a predominance of blue light wavelengths, giving the sky its well-known look. As renowned scientist Bill Nye states, “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). Because of the cones that detect light in our eyes, we find it challenging to perceive the purples that should also exist in the sky.
**What causes a sunset to be red?**
As the sun lowers in the sky, its light must travel through a denser layer of the Earth’s atmosphere. The density of the atmosphere combined with a longer travel distance scatters blue light even more, leaving only red and orange light in 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,” explains Nye. “It’s not magic. It’s science.”
In addition to atmospheric molecules affecting the sky’s color, cloud droplets can also impact the sky’s appearance. If the droplets are similar in size to or larger than the wavelengths, the visible wavelengths will scatter evenly across the sky. This phenomenon is known as Mie scattering, which is why a cloud might seem white. Denser clouds might look gray due to the obstruction of more light, or the cloud may even take on a slightly blue tint if it isn’t receiving direct sunlight, reflecting the sky’s color.
If you’ve witnessed a beautiful sunrise where the sun triumphs over the clouds, it’s a result of Rayleigh scattering and Mie scattering working together. Rayleigh scattering assists in removing all the blues, while Mie scattering highlights the reds in the clouds, crafting a visual spectacle that can be utterly magnificent to behold.
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José Adorno/BGR
