A picture perfect sunset - something that sky watchers everywhere love. So, why are some sunrises and sunsets so spectacular? Lets talk about the science behind these awe-inspiring features.
To start we'll take a step back and think about how the colors of the sky are produced, which has to do with how light is scattered. Scattering refers to the reflection or redirection of light by smaller particles. Rayleigh scattering refers to the scattering of light off of the molecules of air. The particles of air are much smaller than the wavelengths of visible light and therefore air is an excellent Rayleigh scatterer.
Visible light is composed of a spectrum of colors ranging from reds and oranges on one end of the spectrum and blues and purples on the other end. Anyone remember the acronym Roy G. Biv?
|Photo courtesy of: Globalspec.com|
Each color has a different wavelength, with the wavelength of violet light closer in size to air molecules than the wavelength of red light. Therefore, pure air will scattered violet light 3-4 times more effectively than it will for reds. Now think about the blue sky we see every day. The wavelength of blue is close to that of violet, but it still is not scattered as efficiently. Human eyes are more tuned to peak in the middle part of the visible spectrum, or around the color green, which is closer in wavelength to blue. Now, if it weren't for this fact, the sky might actually appear violet to us on a day to day basis!
At sunrise and sunset, the sun is lower on the horizon and the sunlight takes a longer path to travel through the atmosphere than if the sun was directly overhead. The violet and blue hues are more efficiently scattered out through this path, leaving more reds to be seen. Here is an example of this:
|This picture is Figure 1 in Stephen Corfidi's paper: "The Colors of Sunset and Twilight", which is linked below|
|Colors just starting to show up - August 29, 2014|
|Colors intensifying a few minutes later - August 29, 2014|
This post is just a brief introduction to the incredible properties of atmospheric and optical physics. For more details please check this excellent, short paper written by Stephen Corfidi here, which also links to much more information on twilight phenomena.
Here is hoping everyone gets to see some beautiful sunrises and sunsets and can now appreciate the science behind them!
|A couple of sunrise pictures from the NWS Reno office, January 4, 2015. The photo on the left is looking southeast, and on the right looking southwest toward downtown Reno.|