Blue sky

Natural History, May, 2002 by Karl A. Hartman

In describing the origin of the blue light we see when looking at the sky on a clear day, Philip Ball, in "Seeing Red ... and Yellow ... and Green ... and" (3/02), states that "rays from the sun are scattered by atmospheric dust."

The blue color of the sky actually arises from the scattering of sunlight by molecules of air (mostly nitrogen and oxygen), which are smaller than the wavelength of the light, not from dust particles, which are likely to be much larger.

Why doesn't the air all around us appear blue? In a relatively dense medium, such as the air on the surface of the earth, the sidewise scattering of blue light cancels out. However, at high altitudes, where air is less dense, fluctuations in density allow the sidewise scattering to escape without cancellation.

Karl A. Hartman
Kingston, Rhode Island

PHILIP BALL REPLIES: I am grateful to the correspondents who pointed this out. I was aware that molecules in the atmosphere also contribute to the light scattering that makes the sky blue and that they act through a different mechanism (Rayleigh scattering) than does dust (Mie scattering). However, I did not realize that scattering by molecules is predominant. Had I known, I would have recognized that the streamlining of the concepts in the article had led to an oversimplification.