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Colors of the sky

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In our polarized and fragmented times, we appear to have less and less in common. Yet, wherever we live, a good number of universal experiences can bring joy to our lives. Take, for example, a good sunset. How magical is the array of vivid colors splayed across the evening sky as the sun makes its daily descent below the horizon. As gorgeous as such sights are, the moments of beauty are fleeting. In the scope of mere minutes, the sky is transformed from being filled with color to nearly colorless. All around the world, people from every culture have taken joy in admiring these works of art. Staring up at one such display of oranges, reds, and purples, a question struck me; where do sunsets come from?

Sunsets are completely an optical phenomenon, similar to a rainbow. Rays of light emanating from our sun travel for approximately eight minutes and hit the earth. Passing through the atmosphere, the light gets scattered by gasses. This diffusionary process breaks up light into different wavelengths. The various lengths of light waves appear to us as different colors. In order from the longest to the shortest waves, they appear to us as red, green, and blue. The shorter the wavelength, the more quickly the scattering happens. This is why the sky appears blue to us. Much of the blue light gets bounced around in the atmosphere while the reds and greens penetrate the surface, giving sunlight its signature golden glow. 

As the earth spins, the angle of the sun changes. We observe this every day as the sun rises, moves across the sky, and finally sets. As the angle becomes more indirect, the sunlight travels through more atmosphere to reach the surface. Rays of red and green that would reach the surface during midday, suddenly get dispersed along with the blue waves. While the sun dips down toward the horizon, the intensity of the light reaching the surface begins to drop as the amount of light getting scattered in the atmosphere increases. At the apex of these two shifts, the sunset reaches its peak. Dramatic reds, oranges, and purple tones light up the evening sky and clouds with an exquisite glow.

The show is tragically short-lived. As the sun plunges below the horizon, the amount of light drops off and the sunset begins to fade. This doesn’t happen instantly, because light continues to get scattered by the atmosphere and progresses ever so slightly around the curve of the globe. The resulting twilight afterglow fades into darkness as the edge of the sun’s beam gets further and further away.

While we are treated to a beautifully unique sunset each night, colors and vividness can vary in intensity. Some nights display nothing more than a light orange whiff while others explode across the night sky like fireworks made of clouds. Did you know that good sunsets can actually be predicted based on weather conditions? It all comes down to the air. 

The air we breathe is very translucent but not entirely transparent. Water vapor, more commonly known as humidity, clouds the air. These microscopic water particles in the air attract other airborne particles called aerosols to themselves. The aerosols are emitted by both natural and artificial sources. When these tiny particles bind to humidity, they increase in size and start to reduce the translucency of air. Generally, we don’t notice this effect, but it is observable on days when things are a bit hazy looking toward the mountains or other far-off sights. The haze significantly dulls the crisp, vivid colors of a sunset. Thus, lower humidity results in more spectacular sunsets. As a consequence, the most impressive sunsets appear frequently when the weather is colder and the air is drier. To add to the cold weather boost in sunset colors, in the winter, cloud formations tend to be more defined portraying more detail to the show. 

The angle of the earth to the sun also affects the duration of sunsets. Around the solstices, the earth rotates at its steepest angle to the sun. Consequently, the twilight period after the sun drops is lengthened. The farther you are from the equator, the more pronounced the effect. For reference, at the latitude 40 degrees north (roughly as far north as Denver), the solstice sunset is 18% longer than the equinox when the earth is rotating at 90 degrees to the sun. 

So, the next time you are admiring a beautiful display of colors across the evening sky, you can take a moment to appreciate the why behind this natural phenomenon.

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