Question: It occurred to me that we never see green in a sunset except in the apparently rare case of something called a green flash. I have not been able to find online a suitable/understandable explanation for why green does not come through, when we can see all the other spectral colors in the sky at some point. In a rainbow, I can see the thin band of green between blue and yellow, so why not in the sky. — TJR, Harshaw, WI
Answer: See: https://sky-lights.org/2011/10/17/qa-why-there-are-no-green-stars/ for half of the answer. Part of the reason you don’t see green sunsets is the physics explained in that post. The other part is how the eye perceives color with its RGB retinal cones.
When the Sun is setting (or rising) its light undergoes both refraction (bending) by the atmosphere, and a process known a Rayleigh scattering. This produces a gradually changing sequence of color from normal “sky blue” to sunset red. You see that same sequence in a rainbow from violet to red. But unlike a rainbow, sunsets seem to skip the green. Actually, they don’t.
The photo at top shows an oil platform blocking the Sun. With the bright Sun occluded, it’s possible to see a faint green band between the orange and blue. It’s not as intense as the orange or blue, and there’s a good reason for that — the way your eye/brain system perceives colors. When green light mixes with red light, we perceive it as yellow. So the green is there but it gets mixed in with the red. Likewise, above the green it mixes with blue, which we perceive as turquoise. That mixing is caused by natural light scattering in the atmosphere.
The red and blue are at the ends of the visible spectrum and only have to mix with one neighbor — green. Green has to mix with two neighbors, and so ends up the net loser in terms of visibility.
Interestingly, the night sky itself actually is green. On a moonless night you can occasionally see something called airglow. During the day, sunlight dissociates oxygen molecules into atoms. At night, as they recombine, they emit light. For the recombination of oxygen that light is green. It’s usually hard to see with the unaided eye, but shows up on long-exposure photos like this one:
Amazingly, the daytime sky can sometimes change from blue to green. You may have heard the meme that green sky is an ominous portent of tornadoes. It’s not a guarantee, but it is something to pay attention to. Take a look at this photo — definitely green:
Thunderstorms, which can host tornadoes and/or hail, usually happen later in the day when the sun is approaching the horizon. The sunset throws a lot of red light into the sky, as any sunset fan knows. But light under a 12-mile high thundercloud is primarily blue. The sky above the thundercloud is blue, and its blue light is scattered by water droplets in the cloud. When blue light mixes with red light, we perceive it as green.
Your question was specifically about sunsets, so I addressed that first. But I included the green airglow and thunderclouds as additional examples of “green sky” — even though those greens are produced by different physical mechanisms compared to a sunset. I often riff my responses just to teach a bit more science.
Next Week in Sky Lights ⇒ The Moon in 1776