What causes the typical red, pink or orange colors seen in sun-rises and sun-sets?
How come the sky doesn't just turn a darker blue?
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This question is to emphasize my point in my answer to this meta post: http://meta.earthscience.stackexchange.com/questions/1436/lets-get-critical-apr-2015-site-self-evaluation about how we need some simpler questions on the site to attract more new users. – Azzie Rogers Apr 24 '15 at 16:07
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1Just for cross-ref: this is close to being a duplicate of Golden and red colored light even after sunset. There are also one two three similar/related questions over on Physics SE. – Matt Hall Apr 24 '15 at 17:30
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@kwinkunks Yep, it would seem so. I wouldn't have asked this if I had seen that one in the list of 'Questions that may already have your answer'... Do you think they're close enough that this one should be deleted? – Azzie Rogers Apr 24 '15 at 18:33
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As-is, I think they probably are too similar, yes. I'm not very up on the policies there though. – Matt Hall Apr 24 '15 at 20:08
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@kwinkunks: As is, in my opinion, this question is not a duplicate of the question linked to above. If it is, then the first question would need to be edited to cover the scope of this question without changing the intent of the question or somehow expanding the scope of the question to make the current answers less useful or worse, invalid. If have questions about my position, let me know. Thanks. – blunders Apr 24 '15 at 21:05
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There are many references about this on the web, such as Wikipedia and the NOAA site.
At sunrise and sunset the angle the Sun's light makes with the Earth at those locations is low compared with angle the light makes at midday. Because of the low angle, the light has to travel through more of the atmosphere before it reaches the Earth's surface and the eyes of anyone watching either the Sun rise or the Sun set.
The colour of the Sun's light is white, being composed of the colours of the light spectrum: red, orange, yellow, green, blue, indigo and violet. The wavelength of these component colours are all different. Red has the longest wavelength at 650 nm. Each of the other colours have shorter wavelengths with violet having the shortest at 400 nm.
As the Sun's light passes through the atmosphere it get scattered by the air molecules and particles in the air such as dust and aerosols.
Due to a phenomenon known as Rayleigh scattering the shorter wavelengths of light (greens, blues, violets) are scattered first, leaving the shorter wavelengths (yellows, oranges and reds) to travel further.
The greater the amount of particles in the air, dust and aerosols, the greater the degree of scattering and the more red the sunrises and sunsets will appear. This is particularly evident after large amounts of ash have been ejected by volcanoes, after large forest fires and in locations where air pollution, particularly from industrial chimney stacks and internal combustion engines from road vehicles is a major source of airborne particulate matter such as soot or ash and other very small products of combustion
Fred
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2I think I'm missing something here. If blue sky in the day is explained by the blue photons being scattered more and forming a greater proportion of all light in the sky, why would this not be exacerbated at sunrise & set? By the blue day sky logic, the sunset should be really blue. Is it not more to do with a prism type effect, where blue light is bent more than red so we see more red, because the blue is passing high over our heads? But then why is blue light not bent in the same way in the daytime? I guess it's some combination of these effects, and the numbers involved... – Sep 28 '15 at 19:37
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2@Ian during the day the sky is blue because only the short blue and violet wavelengths are appreciably scattered out of the beam of sunlight. As the optical path length increases at sunset the longer wavelengths scatter out of the beam. There is only so much "blue" to be had and much more "yellow" (the sun's peak wavelength). When more colors are being scattered you see the sum of that light, so the turn from blue to warmer colors. Additionally, if there are clouds you often see those directly illuminated by the remaining red/pink/orange colors left in the solar beam. – casey Sep 28 '15 at 19:48
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1I love this picture as the visual is easy to understand. http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.gif Source: http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Also, Ian, the scattering of blue light turns the sun slightly redder, giving it, it's familiar yellow color in the sky. If we had no atmosphere, the sun would appear pure white. – userLTK Oct 06 '15 at 08:40
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