The sky usually is a familiar topic to us, and an aspect that we generally take for granted. However, it is hard to ignore the stunning and sometimes foreboding colors of the sky that occur during a sunset or storm. The sky is constantly changing colors, but what causes the sky to fluctuate between blue, grey, and brilliant reds and oranges? The answer is light and air.
What Factors Determine Color?
The human eye can only see a small fraction of light. The visibility of light is determined by its wavelength, or the distance between two corresponding points on a wave of light. The colors that we are familiar with are part the visible light spectrum, and each color we see coincides to a different wavelength. The largest wavelengths correspond to the reds and oranges and the smallest to blues and purples.
Since light travels in waves, it can be easily scattered if it runs into obstacles. The Earth’s atmosphere contains primarily oxygen and nitrogen, and these small molecules act as obstacles that cause the light to scatter or be absorbed. Since oxygen and nitrogen are significantly smaller than the wavelengths of incoming light, they usually scatter light to the shortest wavelengths- blue and purple. Purple is not seen in the daytime sky because our vision is highest in the middle of the spectrum, which is closest to blue rather than purple.
Sunrise and Sunset
Much like what occurs in the daytime, light wavelengths are scattered into the colorful sunrises and sunsets that we see. However, during these times, the sun is lower on the horizon. This causes the emitted light to travel through more air, which results in more scattering because it encounters a greater amount of molecular obstacles. This means that more blue light is scattered to the point that we can no longer see blue. Instead, we are left with light waves of larger wavelengths- red and orange.
By the time that a sunset is visible, the sun has actually already set. We only see a sunset because of how the atmosphere scatters the light rays. Air plays such a key role in sunsets that in airless environments, such as the moon, sunsets look the same as the normal daytime sky.
A common idea is that higher pollution rates improve the colors in sunsets. However, this is a myth. In areas of high pollution, more molecules are released into the sky. These molecules create more obstacles to scatter and absorb light. Large molecules tend to absorb and scatter all sizes of light wavelengths, which creates a murky and clouded sky. With that in mind, the colors of the daytime sky, sunsets, and sunrises are muted.
During storms, the air has a high content of water molecules. These large molecules can scatter and absorb large wavelengths similar to what occurs with pollution. This results in the grey sky that one sees during a rainstorm.
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Resources: https://news.nationalgeographic.com/news/2013/10/131027-sunset-sky-change-color-red-clouds-science/, https://www.theeagle.com/brazos_life/weather_whys/weather-whys-gray-sky-blue-sky/article_aa566ce2-8118-11e2-b12d-001a4bcf887a.html
Image Resource: Featured Image https://pixabay.com/en/tree-sunset-amazing-beautiful-736885/, https://en.wikipedia.org/wiki/Light#/media/File:EM_spectrum.svg, https://pixabay.com/en/beach-north-sea-sea-sunset-water-2179624/, https://pixabay.com/en/clouds-summer-storm-clouds-form-194840/