Why Is the Sky So Blue in Autumn?

Autumn color surrounds a pagoda in Fujiyoshida, Japan as Mount Fuji looms in the distance. (Photo: lkunl/Shutterstock)

The color of the leaves isn't the only thing that changes in autumn. Have you ever looked up on a crisp fall day and noticed how brilliant and clear the blue sky is? That's not just your imagination — the sky really is more blue, and all because of science.

To understand why the sky is so especially vivid in autumn, you first need to understand why it's blue in the first place.

"Why is the sky blue?" is a classic query posed most often by curious young children, and unlike the many other grand mysteries of our universe, we know the answer to this one, thanks to a gentleman named John William Strutt. This 19th-century physicist won a Nobel Prize in 1904 for discovering the element argon, but what really cemented Strutt's place in the history books is his discovery of Rayleigh scattering. Named for Strutt's inherited status as the third Baron of Rayleigh, this phenomenon explains how light scatters into different colors based on the molecular contents of the atmosphere.

Let's go over the basics of how blue skies work: Light from the sun is made up of many colors, which manifest in different wavelengths. For example, red light has the longest wavelength and, on the other end of the spectrum, violet and blue light have the shortest wavelengths. When light passes through the Earth's atmosphere, it comes up against thick layers of gas molecules and dust particles. These tiny atmospheric bits are closer in size to shorter wavelengths, which is why blue and violet light scatter more easily. The result is our beautiful blue sky.

But wait! It's important to mention that although we see a blue sky, the truth is that it's actually violet. The reason we perceive the sky as blue instead of violet is due to the physiology of our eyes, which are more sensitive to blue.

Yellow and orange autumn leaves contrast against a bright blue sky. (Photo: nohanka/Shutterstock)

So now that we know why the sky is blue, it's time to go back to the original question — why does the sky appear extra blue as we descend deeper into autumn? There are several reasons for this.

The sun is positioned lower in the sky.

As the days get shorter, the sun's path across the sky sinks lower to the horizon. This bumps up the amount of scattered blue light that reaches our eyes on the planet's surface.

"The sun is no longer directly overhead and more of the sky is significantly angled away from the sun," according to Wildcard Weather. "The Rayleigh scattering directs more blue light towards your eyes while the indirect sunlight decreases the incoming levels of red and green."

Less humidity means less haze and clouds.

As our summers continue to break soaring temperature records, there's something so comforting about the seasonal reprieve that autumn brings. Not only are temperatures milder, there's also less humidity across the board. Since the air isn't holding as much moisture, clouds don't form as easily and haze doesn't clog our urban centers. The result is a crystal clear view of the azure expanse above.

The warm hues of fall foliage naturally complement the blue sky.

If you've ever made a color wheel for an art class, you'll know that blues and oranges are complementary colors. As direct "opposites" of each other, the gold, orange and red leaves of autumn pop beautifully against the already brilliant blue sky.

Clear autumn skies over Lake Millstatt in Austria. (Photo: Yauheni_M/Shutterstock)