Why the Sky Is Blue — and Where to Watch That Change the Most Dramatically at Sunset
On most days and in most places, the sky above the surface of the Earth appears blue. But think about it: Is the sky really always that color? What about gray days, or rosy-fingered dawn, or “the red sky at night” from that one sailor rhyme?
The answer to “Why is the sky blue?” is functionally the same as "Why do colors exist?" Color is light as we are able to perceive it. The sky is so many colors (chief among them, blue) because it is suffused with light.
Visible light is a type of electromagnetic radiation — a narrow slice of a broad spectrum of energy that includes radio waves, microwaves, ultraviolet light, x-rays, and gamma radiation — that the human eye can perceive through sight. White light, which the sun emits, is a combination of all the different lengths of electromagnetic waves we are able to see.
Color appears when our eyes register only some — but not all — of the wavelengths we can perceive. Red light, for instance, is the slowest wave visible to us: energy that moves in long and undulating ripples. Blue, on the other hand, is the fastest: energy that shivers in a choppy and rapid rhythm.
The sky changes color as white light from the sun hits the Earth’s atmosphere and reacts in different ways. Light waves —along with the rest of the electromagnetic spectrum — will travel in a straight line unless they hit something. Waves can be reflected (as with a mirror), bent (as with a prism), or scattered (as with the sky).
Though the sky (a.k.a air) often looks invisible to our eyes, it’s very much a positive presence, a shifting and complicated mix of gases and particles. White light, to get from sun to our eyes, must bounce around a maze of countless molecules in our atmosphere first.
Bouncing the most are the already jittery blue wavelengths. Because of its short, small peaks and valleys, it’s more likely than other waves to hit an obstacle and be scattered in all directions. As a result, the sky directly above any given point on the surface of the Earth will look more blue than the sky observed above the distant horizon. Towards the top of the atmosphere, it's blue light that’s most visible because it’s initially the first type of electromagnetic wave to scatter.
Towards the bottom, the whole spectrum of visible light has filtered through considerably more air, and much more of it has scattered. With not just blue waves but red, orange, yellow, green, indigo, and violet waves bouncing around, the mixed up light appears white again: a combination of them all.
Whether you are looking at a midday sky that’s a perfect robin’s egg blue, a deep, cotton candy-streaked sunset, a dramatic red dawn, or a gray afternoon — it’s all a trick of the light.