The Science Behind Sunsets | Sydney Hecht

Although we’re fortunate to live in Southern California, where blue skies are often seen year-round, we can all attest to the fact that this winter, in particular, has been a bleak one. Grey skies, windy afternoons, and rainstorms have plagued the weather forecast for the past month, and have made me, for one, increasingly excited for summer.

Still, I always find the colder months trump the summer months in one particular regard: their consistently stunning, vibrant, beautiful sunsets.

The Science Behind Sunsets:

First, where do the colors of the sunset come from? How can we see the sky’s bright orange, purple, and pink hues? Like sound, light travels in a variety of wavelengths. Each color of the visible light spectrum corresponds to a different frequency, with violet having the shortest wavelength and red having the longest. In a process called “Rayleigh scattering” (or more commonly known as “light scattering”), these rays bounce off the oxygen and nitrogen particles that fill the air around us, in turn changing the direction of the light. This phenomena is what causes the sky to appear blue to the human eye–since blue and violet have the shortest wavelengths, they scatter more easily. Therefore, there are more of these waves to reflect back into our eyes, giving the sky its trademark blue tint.

This is why sunrises and sunsets take on a more reddish-orange hue. When the sun is closer to the horizon, the rays of visible light must pass through a thicker level of the atmosphere. This air is more densely populated with air particles, causing light scattering to occur at a more frequent rate. Therefore, the shorter light waves (blue and violet light, especially) are too scattered for them to pierce the dense atmosphere–only longer wavelengths, that correspond to the colors red and orange, are able to reach our eyes, creating the colorful skies we see at both dawn and dusk.

Although there are many factors in determining a “good” sunset, intensity is generally another that makes the sky particularly stand out. The sky’s vividness is determined by the proportion of rays that reach the human eye–when the waves are scattered less frequently, their corresponding colors can be seen more easily. This is why the sky becomes dimmer as the sun sinks lower–since the atmosphere holds more air particles as the sun sinks lower, there are fewer transmittable light waves. Additionally, the types of air particles in a respective area can enhance (or inhibit) the vividness of a sunset. Larger and denser particles, like dust and smoke, absorb more light than naturally occurring ones like oxygen and nitrogen. Since a greater amount of light is soaked up by these pollutants, the colors of the sky in more contaminated areas may appear more muted to the human eye. Clean air is primarily responsible for vibrant, colorful skies.

Clouds also play a huge role in determining the vibrance of a sunset. Rather than blocking the colors of the sky, a notion that many believe to be true, they actually enhance them. As the sun sets, the light waves reflect off the bottom of the clouds and back towards the ground, ultimately making for a more vibrant sky. High and mid-level clouds, specifically cumulus and cirrus clouds, are able to reflect the most sunlight, and therefore, are responsible for the most noteworthy sunsets.

Now, to answer our question: why are winter sunsets, in particular, so beautiful? The reason for this, put simply, is the better quality of air molecules during the winter months. Similar to the way pollutants subdue the colors of the sky, the warm, humid, summer months cause the air particles to absorb more water, thereby enhancing their capacity to inhibit light waves from traveling to our eyes. Rather, the less-water-absorbent air particles in the winter months facilitate less light scattering, and in turn, make the sky more vibrant. Since the air itself is cleaner and crisper, the sky takes on a bolder hue.

Hopefully, if you are an avid sunset-goer or a striving meteorologist (or both!), you’ve learned something about what makes the sky so beautiful each night. As this winter draws to a dramatic close, make sure to take the time to appreciate the stunning sunsets we’re sure to see in the coming couple of weeks.