Dear Tom,
I have actually seen the so-called “green flash” on the setting sun. What causes it?

Ronald Ansouel, Chicago

Dear Ronald,

Congratulations on your sighting! You have observed something that few of us have ever seen. The “green flash” is a very brief flash of green light that emanates from the upper rim of the rising or the setting sun. 

Incoming sunlight is bent, or refracted in its passage through the atmosphere. This bending is greatest when the sun is positioned on the horizon because the rays must travel through more atmosphere. Blue and green colors are shorter wavelengths within the spectrum of sunlight and undergo greater bending and scattering. Thus, they are the first colors to appear on the upper rim of the rising sun and the last to appear on the upper rim of the setting sun. The atmosphere scatters out most of the shorter wavelength blue light, leaving green as the shortest wavelength to make it through to our eyes.

On occasion, the temperature structure of the atmosphere causes it to act like a magnifying glass, enlarging the green rim and making the brief green flash visible.