Why is the Sky Blue? An Easy Answer to an Old Question.

In our lives, we’ve all looked up and wondered, “why is the sky blue?” After all, color aside, we’ve all seen the sky every day since we were born. And, while the answer involves basic chemistry, physics and meteorology, the answer is VERY simple! Let’s break it down…

Earth’s atmosphere

why is the sky blue earth's atmosphere

Indeed, we enjoy a safe life under our protective atmosphere on Earth. But, what actually makes up our atmosphere? Actually, Earth’s atmosphere is mostly nitrogen gas (78%) and oxygen (21%). However, it also contains water vapor, ice crystals, dust and soot. In fact, our atmosphere even contains salt from evaporated ocean water!

Not to mention, our atmosphere changes, depending on your location. And, it’s thickest near the bottom, much thinner near the top.

Most importantly, the various materials in our atmosphere affect light, and how we see it! But, we’ll discuss that below.


why is the sky blue rayleigh scattering

Simply put, light is energy traveling in waves, or radiating. And, the powerful of light’s energy depends on its wavelength. In other words, the shorter the wavelength the more powerful, the longer the wavelength the less powerful.

To human eyes, light appears white, or colorless. However, light contains red, orange, yellow, green, blue, indigo and violet colors (“R.O.Y.G.B.I.V.”). But, until something causes the colors to separate out, it remains white.

Actually, we see the colors separated through a prism, like the album cover of Dark Side of the Moon. In fact, rainbows are light with separated colors!

Light travels faster than anything in the known universe, in a straight line unless it bumps into something. Don’t worry, this will also become important below!

Light and Earth’s Atmosphere

why is the sky blue rayleigh scattering diagram

Technically, light can interact with Earth’s atmosphere in different ways. And, the interactions depend on the light’s wavelength (power) and what it interacts with (atmospheric material).

Materials like water droplets and dust are much larger than light waves. Therefore, light will completely reflect in all directions and remain white.

But, gas molecules, like nitrogen are far smaller than light waves. Therefore, nitrogen will absorb certain colors from the light. Eventually, the molecule spits the color back out in various directions. This absorb and scatter process is called Rayleigh Scattering.

Yes, all colors of light can be absorbed. However, colors with higher energy (colors on blue side) are absorbed much more than low energy colors (red side).

So, why is the sky blue?

why is the sky blue rayleigh scattering

Finally, why is the sky blue? As light enters our atmosphere, low energy colors (red, orange, yellow) are mostly unaffected. Whereas, high energy colors (violet, indigo, blue) are absorbed and re-scattered. Therefore, no matter where you look in the sky, it appears to be blue!

Then, nearing the horizon, the sky appears much more pale and white. Now, from this angle, light must pass through much more atmosphere. As a result, much more light is either reflected, or scattered. This causes a much less one-color sight in the skies!

But, what about sunsets and sunrises?

why is the sky blue rayleigh scattering sunsets

Lastly, as our Sun approaches the horizon, its light must pass through much more atmosphere. Therefore, blue color is even more scattered, while red is still rather unaffected. As a result, colors like red, orange and yellow make it straight to your eyes.

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Don’t forget to check out Astronimate’s video “Why is the Sky Blue?”

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