Why far-away mountains look blue

Q: If you are standing in the foothills of a mountain, you see greens, yellows, oranges, and many other colors — but, if you are about three to seven miles away, it turns blue and all the other colors are gone. What causes this bluish mountain? Is it a blue haze or is it a scattering of light reflected from the mountain?  Michael, Corona, California

Mountains look blue for the same reason the sky looks blue. Very distant mountains look a slightly lighter blue than medium distant ones. Photo courtesy of NOAA.

A: We see blue low-sky light when we are far from a mountain, and that makes the mountain appear blue. The farther away the mountain, the brighter blue it appears. By low-sky light, I mean the layer of air between us and the mountain.  Low-sky light looks blue for the same reason the sky, in general, looks blue.

The sky is blue because tiny air molecules of oxygen and nitrogen, water molecules, and dust motes interact with light. The small sizes of these particles means that high-frequency light (like, blue) is much more likely to interact than low-frequency light (such as red). The interaction scatters blue light in all directions. Consequently, we are more likely to see blue light than any other color. Thus, the sky looks blue.

Distant dark mountains reflect little light to our eyes. Our eyes receive much more light from sunlight scattered by tiny molecules between us and the mountain. That scattered sunlight is blue. So, it isn't a scattering of light reflected from the mountain that makes the mountain appear blue, but rather a scattering of light between us and the mountain.

Further Surfing:

WeatherQuesting: Why the sky is blue

Rod Nave, Georgia State University: Hyperphysics, Blue sky

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