The Science of Sunsets

2017-06-12 12:30:15.000 – Julia Moreland, Summit Intern

Sunsets and sunrises are lovely phenomena indeed. The coming and going of the days and nights are incredible in it of themselves, but there is something about the painted flames of color seeming to emerge out of nowhere that makes people stop in their tracks. You may have found yourself with strangers on the side of the road, on the beach or upon a mountain gazing at the deepening reds and oranges, exclaiming your admiration for the beauty, despite never having met them before in your life. That really is a wonderful thing that nature does – we all forget about the darker parts of the world and focus on the light.

As a scientist, I am intrigued not only by the natural beauty of sunrises and sunsets, but also the science behind the colors. I have been fortunate enough to witness many sunsets during my short time on the summit so far, and all of them have been different in many ways, especially from ones I’ve seen in the valley.

There are many factors that determine the types of sunsets that we see, including location, seasons, cloud type, and smoke, among others. To begin with the typical trends, sky colors are produced as a result of the scattering of light by air molecules in the atmosphere. Because air molecules are much smaller than the wavelengths of light, they have proven to be very effective at scattering light (known as Rayleigh scattering). However, due to these molecules being closer in size to the wavelength of violet, and the sensitivity of the human eye to blue light, we see a blue daytime sky. As it nears the end of the day, the lovely colors often seen at sunset can be attributed to the fact that at such a low zenith angle, the sun’s rays must pass through a thicker amount of atmosphere before reaching the earth. This increases the amount of violet and blue being scattered out, causing the observer to see a more reddened color – the longer end of the spectrum with regard to wavelength (NOAA).

Many sunsets with a crisp, clear sky can be remarkable, as you are able to see the gradient of color from the horizon to the darkening night sky. However, when a cloud runs ablaze with the tip of the sun on the horizon, it can appear as if the sky was magnificently painted in a range of acrylics. Cloud type/height is one of the most important factors, as they act as a multi-layer backdrop for the colors of sunset and sunrise. The most vivid colors will often be seen in the highest clouds, typically cirrus and altocumulus, as the light has not had to pass through the lowest layer of the atmosphere (known as the boundary layer) which contains large particles of dust and haze. In the case that the lower cloud deck is lighting up with vivid colors, this is an indication of a dry, clean boundary layer with much smaller particles. Clouds such as altocumulus often exist at inversions, where there is typically a sizable amount of wind shear. This can create a wavelike effect that can change dramatically and create a wonderful display as the sun rays hit them (NOAA).

This is where the phrase “Red sky at night, sailor’s delight; Red sky in the morning, sailors take warning” stems from, and there is actually quite a bit of truth to this old saying. In the case of an evening sky

covered heavily with mid to high level clouds displaying the vivid oranges and reds, it is likely that weather and instability in the upper atmosphere is likely to dissipate, indicating more stable air/high pressure moving in. If you see this at sunrise, however, it likely means that this stability has already passed, means there is likely weather on the way, moving in from the east.

It is not only clouds within the troposphere, the first layer of the atmosphere that can contribute to the colors. Particles in the stratosphere can also induce some intriguing optics. In the case that there is volcanic ash/smoke present in the atmosphere, vibrant colors can be observed, especially after the sun is set. Typically these particles are invisible during the day due to scattered sunlight, but within the half hour after sunset, the stratosphere is still illuminated and allows these upper level clouds to appear. This effect can be magnified if a recent eruption from a volcano spews ash into the upper atmosphere.

One of the most rare and interesting aspects of the rising and setting of the sun is the famous “green flash.” It is a phenomenon that occurs primarily at sunset and over the ocean, where temperature inversions are frequent. Just as the top of the sun’s disk sinks below the horizon, a fleeting green hue may appear. This is due to light being refracted by the atmosphere – the blue light is scattered, while yellow and oranges are absorbed, so all that is left are the red and green wavelengths. This will make it appear as if there are a red and a green sun stacked on one another, leaving the green flash to appear just as it dips below the horizon (Library of Congress).

Julia Moreland, Summit Intern