by Captain Kirk Greiner
As young cadets sailing aboard the Coast Guard Academy's square rigged bark EAGLE, the upper classmen told us "swabs" about the "green flash", a phenomenon observed when the sun sets. On a clear night, as the upper limb of the sun sets below the horizon, sometimes there is visible a "green flash". At first, I thought it was a tall tale being told to see how gullible we were. However, it turned out there is such a phenomenon as discussed in MAC Newsletter Spring 1989. (For Web readers, click on the citation to go to the prior article.)
With some surprise, I received an e-mail from Jeffrey R. Simpson who had come across the MAC homepage and asked what we could tell him about the "green flash." After referring to the previous MAC Newsletter article, he wasn't content with that and continued his research further. Set forth below are the results of his research which may be of interest to those seamen amongst us.
Thanks for a good description of the phenomenon known as the green flash. You might want to add the following information. There is a dispersion of the refracted sunlight due to density variations in the atmosphere with lower frequencies (red) being refracted less. This would imply that as the sun sets below the horizon higher frequencies (blue) would be seen longer. Of course sunsets are not known for their brilliant shades of blue. So where did the blue go? The answer is scattering (specifically Rayleigh scattering). Sunlight incident on the atmosphere is reradiated by motion of particles in the atmosphere (dipole radiation). There is a strong frequency dependence on this reradiation (frequency to the fourth power) and hence blue dominates. As the sun sets, light has to travel a longer distance through the atmosphere to reach you and in so doing all the higher frequencies are scattered out. The remaining frequencies are dominantly red (sunsets) and as the sun sets further and red is no longer refracted enough to be seen ... green flash.