Sunsets come in all colors. Well, not exactly. I’ve never seen a green sunset, but on a day when the smog was very bad, I saw one that looked greenish-gray. I suspect most of the green was the color of the smog itself. But when you limit yourself to “natural” sunset colors, violet and/or indigo are the most uncommon.
The reason for this is the way our atmosphere refracts and scatters light. For a quick review on scattering see my column on Why Most Sunsets are Red. Refraction is the bending of light rays as they move from one medium (outer space) into another (Earth’s atmosphere). It’s the same thing a prism does to light. The individual colors are dispersed, with violet changing direction the most, and red the least.
This photo was taken about 30-45 minutes after sunset, looking toward the west from Arizona. You can probably tell the Sun is already below the horizon. With this geometry, there’s no direct rays of sunlight reaching your eyes. The reds, oranges, and yellows are now coloring a sunset in California. I saw them 30-45 minutes ago.
Those violet and indigo colors are being refracted by the atmosphere over California and sent our way. This requires clear upper air and just the right Sun angle to happen, and it never lasts as long as a red sunset. The undersides of the clouds are reflecting those violet and indigo rays to my camera. The colors disappeared in less than 10 minutes, but a photo lasts forever.
You might well ask: Why aren’t there green sunsets? That’s a topic for a future column, where I’ll discuss a very rare phenomenon known as the green flash. If you’re impatient, look here.
As an enthusiast deeply knowledgeable about atmospheric optics and natural phenomena, I'll address the concepts mentioned in the article about sunsets and their colors.
Firstly, the colors of sunsets are a result of how Earth's atmosphere refracts and scatters light. This scattering, explained by Rayleigh scattering among other phenomena, is responsible for the vibrant colors seen during sunset, where shorter wavelengths like blue and violet get scattered more, leaving longer wavelengths such as red, orange, and yellow to dominate the sky.
The article rightly mentions that while green sunsets are rare, they can be influenced by atmospheric conditions like heavy smog, where the presence of pollutants may give a greenish-gray appearance. However, natural green sunsets due to atmospheric effects alone are extremely unusual.
Refraction, a key principle discussed in the article, occurs as light passes through different mediums, in this case, from outer space into Earth's atmosphere. Similar to a prism separating light into its constituent colors, the atmosphere disperses sunlight during sunset, with violet bending the most and red the least due to their respective wavelengths.
The specific mention of violet and indigo being the most uncommon colors in sunsets is accurate, attributed to their shorter wavelengths and the angle at which they refract through the atmosphere. These colors become visible during specific conditions, often requiring clear upper air and a particular angle of the Sun.
The description of the photograph taken after sunset, where the Sun is below the horizon, emphasizes how indirect rays of sunlight still illuminate the sky, showcasing colors like reds, oranges, and yellows that were visible in a California sunset about 30-45 minutes before the photo was taken in Arizona.
Moreover, the article touches upon the fleeting nature of violet and indigo hues in sunsets, mentioning how they vanish relatively quickly compared to the more common red sunset colors. The role of cloud undersides in reflecting these less common colors to the observer is highlighted, indicating their transient nature.
Lastly, the article hints at a future discussion on the rarity of green sunsets, teasing the topic of the green flash—a fascinating optical phenomenon occurring just before or after sunset or sunrise, where a green spot is briefly visible above the upper rim of the solar disk.
Overall, the article succinctly delves into the physics of sunset colors, refraction, scattering, and the atmospheric conditions that contribute to the vivid hues observed during these natural phenomena. It effectively outlines the rarity of certain colors like violet and indigo while hinting at the exceptional nature of green sunsets and the intriguing green flash phenomenon.
