"It is almost impossible to watch a sunset and not dream." – Bern Williams

With all this snow and ice, I definitely wanted to steer clear from writing about the science of snowflakes and ice cream, so I chose a subject that evokes warmth (think California, beaches, romance): sunsets!

California (check).  Beach (check). Romance (probably there). Just need a red sunset!

Frank Sinatra once sang, “What makes the sunset?” I’m sure many of you have sang (well, most likely just asked) the same question when you were younger and know it’s due to the earth’s rotation every 24 hours. However, here’s a more fun question to ask (and dream about): “What makes the sunset colorful?”

To answer this, let’s start off with a more basic question: why is the sky blue? This phenomenon can be explained by Rayleigh scattering (named after the physicist Lord Rayleigh), which is the scattering of light by tiny, tiny particles. These particles aren’t necessarily dust particles, which many people incorrectly assume are responsible for the scattering of light. In fact, they are mostly atoms and molecules (everything you see is made up of this SUPER tiny stuff) in gases such as oxygen and nitrogen. Thus, the sky during the day looks blue because the atmosphere scatters light with shorter wavelengths (such as blue) much more than light with longer wavelengths (such as red). This effect is known as diffuse sky radiation. In fact, the resulting “sky blue” color that the human eye perceives is actually an “average" of the scattered colors with shorter wavelengths, mostly blue and green. Although violet light has a shorter wavelength than blue light, it is not a major component of the solar radiation spectrum and is not easily detected by the human eye. Side note: as a kid, I’d always look out the window during car rides and wonder why the “sky” looks black in space. I later learned in school that the lack of atmosphere in spaces prevents light from scattering, thus the blackness.

No, the sky is not blue because of the reflection of the sea! Saaremaa, Estonia.

Ok, now back to sunsets. During the latter part of the day, a majority of the light we see is coming in is going in the direction of the Earth's spherical surface (aka tangential), so it has to travel a much longer path through the atmosphere to get to your eyes than at noon when the Sun is directly overhead and travels a much straighter path. Therefore, the blue light is scattered out, and the remaining light at longer wavelengths, such as yellow and orange, is what you eventually see. Sunrises can also be explained the same way; however, sunset colors are usually more vivid because there are more particles in the evening air than in the morning air. Furthermore, if the atmosphere is more polluted with dust particles, the sky will be redder. Likewise, sunsets over the ocean are more orange most likely due to salt particles from the sea.

Sunset at True Blue, Grenada. Photo credit: Aparna Balasubramanian.

Sunset at Grande Anse Beach, Grenada. Photo credit: Aparna Balasubramanian.

I love taking photos of sunsets, and I’m sure most you do as well. I’ll share two that I took (the first one is right in my backyard, and the 2^nd is from Tallinn, Estonia). See, even during winter you can have some warm-looking sunsets :)

Sunset over the pond in my backyard.

I was lucky enough to even be awake for a sunset in Tallinn! Tallinn is located waaaay up North. During the summertime, especially mid-late June, they get about 22-23 hours of daylight there! It was so cool to go grocery shopping at 10 pm when it was still broad daylight, and to go to bed at 2 am with the sun still blazing! For that, I can thank Earth's position during its revolution around the sun (http://en.wikipedia.org/wiki/Summer_solstice)!

View from our apartment in Tallinn Estonia. Sunset behind the Alexander Nevsky Cathedral.

Also, check out Aparna's awesome pictures above from Grenada! Totally dreamy.

So the next time you make your dramatic ride off into the sunset, you can thank molecules for the breathtaking scenery!