Question: We just got back from a summer road trip to the Blue Ridge Mountains in North Carolina. I have to say, they really did look blue, but I know that’s not their real color. Our tour guide said it had something to do with light in the atmosphere, but I didn’t really understand his explanation. Could you clarify this please? — JB, Los Angeles, CA
Answer: Easily. But you don’t need to travel to the Blue Ridge Mountains to see this effect. All distant mountains appear blue for the same reason. The image above was taken in Arizona from my home north of Phoenix. The closest hills are about 400 meters (1/4 mile) south of me, and show the typical brown-green coloring of our desert vegetation.
The most distant mountains visible are part of South Mountain, about 70 km (44 miles) to the south. They appear distinctly bluish. All the mountains in this photo have the same vegetation, but the farther away they are, the bluer they appear. For comparison, click on the thumbnail below showing the “official” Blue Ridge Mountains. Their natural vegetation is more green, but they also show the effect.
The cause is closely related to something called atmospheric extinction, discussed in my Feb 4, 2013 post. The effect is due to Rayleigh scattering of light in our atmosphere. It’s worth a full-size graphic to explain this important phenomenon, since it’s the reason our sky is colored blue. Incoming sunlight is “white” (a mix of all colors). Shorter wavelengths of light (blue & violet) scatter strongly off the nitrogen molecules that compose 78% of our atmosphere. The green, yellow, orange, and red wavelengths pass essentially unimpeded all the way to the ground.
The result, as the graphic shows, is a lot of blue and violet light bouncing around in the atmosphere. The more atmosphere between you and a mountain, the more blue that mountain will appear. Not so on Mars, with its 95% carbon dioxide atmosphere, where Rayleigh scattering produces a salmon-colored sky.
The law of Rayleigh scattering states: When the wavelength of the light is comparable to (or less than) the diameter of the molecules, then those waves will be preferentially scattered. Longer waves will preferentially diffract around the molecules.
The effect is not only visible on distant mountains. You can see it on any distant object, like skyscrapers in a city skyline. All you need is a sufficient amount of atmosphere between you and the object, and the scattered blue and violet light will “overwrite” the object’s true colors.
Next Week in Sky Lights ⇒ Sun Pillar Sunset
Sun is right below the horizon and the mountains are needed, we won’t be able to easily distinguish that the blue sky actually depends on the mountains.
Not sure what you’re asking here. The Sun is not below the horizon. That photo was taken about an hour before sunset. You can see that from the shadows on the mountains. And it’s not that the “blue sky depends on the mountains” … rather, the blue mountains depend on the sky.
I have always wanted to understand this. Did we cover this in your physics class many years ago? 🙂
What I did not know, but makes so much sense, is that the light scattered is about the relationship between the wavelength and the size of the prevalent atmospheric molecule. Cool!
We did indeed. Remember the “wave tanks” we used to study general wave phenomena? By placing an aluminum disc in the water, and then adjusting the wavelength of the wave generator, we demonstrated how the amount of reflection vs. diffraction was a function of wavelength vs. object diameter.