Why Distant Mountains Appear Blue to the Naked Eye

A hiker surveys the view from the summit of Grandfather Mountain in the aptly named Blue Ridge Mountains of North Carolina. Seth K. Hughes/Getty Images/Image Source

Coors Light beer commercials get a lot of mileage out of the brand's color-changing labels. The beer is sold in cans and bottles decorated with special "thermochromic" ink. As the temperature shifts, so does the ink's coloration. At about 48 degrees Fahrenheit (8.8 degrees Celsius) or colder, the mountain-shaped Coors logo turns blue.

Since Americans tend to like their beer chilly, this is a helpful signifier: "When the mountain turns blue, it's as cold as the Rockies." Or so sayeth Coors.

But why does the tiny mountain turn blue when it's chilled? Why not pink or yellow or forest green?

If you've ever seen a mountain range in real life, then the choice will make sense. Distant mountains naturally tend to look bluish. Australia's Blue Mountains and the Blue Ridge Mountains of the eastern U.S. weren't named at random, you know.

Indeed, on a clear day, it can be hard to tell where some far-flung mountain peaks end and where the sky begins.

Mister Blue Sky

The sky itself usually appears blue during the day thanks to atmospheric distortion and the limits of human eyesight. It's a phenomenon called Rayleigh scattering.

Our sun, the magnificent star we all depend on, gives off white light. Rays of sunshine appear white because they mix together all the colors of the rainbow. We're talking red, orange, yellow, green, blue, indigo and (last but not least) violet.

All of those colors travel at their own, distinct wavelengths. Red light has the longest wavelength among them; violet light has the shortest.

Sunlight requires an average of eight minutes and 20 seconds to reach the Earth. Things get interesting once it hits our atmosphere, which is loaded with unimaginably small air molecules. Even wavelengths of visible light dwarf the tiny air molecules.

Light with shorter wavelengths is more likely to hit the air molecules and get scattered around by them, bouncing like a Ping-Pong ball from one molecule to the next until it eventually hits our eyes from any number of possible directions.

And wouldn't you know it? Blue light has one of the shortest wavelengths in the entire visible light spectrum, which means blue colors scatter most in the atmosphere.

True, violet wavelengths are even shorter. But the sun releases less violet light than blue light to begin with, and the human eyes detect blue more easily.

A Colorful Divide

This scattering of so much blue light in the atmosphere, combined with unequal blue light output from the sun and the biases of our vision, answers that age-old question: "Why is the sky blue?"

We have this same process to thank for the bluish tint of distant mountains.

When you gaze at a faraway summit, there's a whole lot of atmosphere sitting between your eyeballs and the actual mountain. The quantity will only increase with distance. More air means more air molecules, which means more light-scattering.

As the space between you and your favorite mountain widens, the latter gets bluer and fainter until — finally — it disappears from sight. That's why when we look at mountains far off in the distance, they appear to look blue.

By the way, this phenomenon also applies to tall buildings. I live in northeast Queens, New York, and the very best thing about my morning commute is a spectacular view of the blue-draped Manhattan skyline.

It almost makes up for the traffic jams.