Under the right conditions, the upper edge of the setting sun will blaze bright green just before dipping below the horizon. What causes these "green flashes"? The answer is more complicated than you probably realize.
Above: A green and blue flash photographed during a sunset visible from Teide Obervatory at Tenerife, Canary Islands, Spain | Photo by Juan José Marzano, used by permission
What causes green flashes? The simplest explanation I can muster is that green flashes are the result of Earth's atmosphere refracting, i.e. bending, light – the same atmospheric refraction and scattering effects responsible for a sunset's prototypically ruddy palette. The atmosphere bends shorter wavelengths of light more dramatically than longer ones, an effect known as atmospheric dispersion.
"The setting Sun's disk is made up of light of all colors," explains M. M. Dworetsky of University College London's Department of Physics and Astronomy. "Green and blue light [shorter wavelengths] are refracted by air slightly more than red light, so the disk actually consists of a flattened red disk, with a yellow disk slightly above it, a green disk above that, and blue and violet disks at the top."
When the sun sets, these bands of color dip below the horizon in this order, with red disappearing first and violet last. Why then, you may ask, have you not observed a violet, blue, or green sunset? The answer, expertly summarized by San Diego State University's Andrew T. Young, pertains to an effect known as atmospheric extinction:
Both air molecules and aerosol particles scatter the shortest wavelengths most strongly (which is why the sky is blue: the strongly-scattered blue light goes in all directions, so we see it when we look anywhere in the sky). At the horizon, the path length through the air is very long, and the shortest wavelengths are almost completely removed.
Scattering by molecules alone is not quite enough to make the shortest wavelengths invisible; so if the air is very clear, violet is the last color seen. But usually there is enough haze in the air that violet, and even blue, is completely removed, so that green is the last color seen at sunset, or the first at sunrise.
When atmospheric dispersion and atmospheric extinction conspire with a clear, unobstructed view of a distant, low-lying horizon, conditions are prime for observing a green flash (and, occasionally, flashes of color at even shorter wavelengths). The following series of images, photographed by Göran Strand from Frösön island in northern Sweden, preserve the fleeting appearance of a green and blue flash, as seen during the final seconds of sunset:
Above: A green and blue flash, as seen during the final seconds of sunset from Frösön island in northern Sweden | Photo by Göran Strand [FB / T / IG / YT], used by permission
Now. I mention above that this is the simplest explanation for green flashes that I can provide. This is because any additional description would require me to get into mirages – a crucial ingredient in the production of green flashes that I have omitted entirely from my explanation.
You see, there are various kinds of mirages, and each kind gives rise to a different kind of green flash. "That means a separate explanation is needed for each kind; and you have to understand the mirages before you can understand the flashes," writes Young, an expert on photometry, planetary atmospheres, and, of course, green flashes. But before you can even begin to understand the mirages, Young explains, one must familiarize oneself with the important difference between astronomical mirage phenomena and terrestrial mirages – and for that, you really need an entirely separate introduction to mirages, and maybe a few simulations to illustrate some common green-flash forms.
And this, dear reader, is where I defer to Young's vastly superior knowledge on the subject of green flashes.
I have not written a primer on mirages, nor have I created any simulations of green flashes. Young, however, has. In fact, his Introduction to Mirages and Simulations of Green Flashes are but two outstanding components to a deep, dizzying network of over 100 web pages devoted not only to characterizing this elusive optical phenomenon, but characterizing it accurately – incorrectness, inexactness, and incompleteness being just three of several major problems Young identifies within the mind-bogglingly vast body of literature devoted to green flashes. So extensive is Young's Companion to Green Flashes that the guide, itself, comes with a guide. Said guide includes a table of contents, a glossary, and an alphabetized index.
I invite you to explore the extent of Young's Green-Flash Nexus for yourself. Well-organized it is not, but what it lacks in planning it more than makes up for in thoroughness. Seriously. It's incredible. Go have a look.