Sunrise and Sunset Visual Differences
Date: Spring 2012
It seems that sunrise and sunset don't look symmetric. I mean that sunsets tend to have much redder skies and sunrise is usually a bit gloomier. If you see a picture, many times you can tell if its sunrise or sunset, even though I would think they should like identical, except that the sun is either going up or going down. Why do they not appear the same but in reverse?
Funny you should ask as a paper just arrived which is sure to have the answer and I will read it now…….
OK, the morning sky, and the sky in general, is blue due to Rayleigh scattering [which affects short wavelengths the most] of the sun light by air molecules and other microscopic particles.
In the evening, because the atmosphere is warmer, there are more water molecules, which are larger than air molecules, and theses scatter the long-wavelengths giving the distinct orangey-red sunset sky. [Journal of Vision (2012) 12(3):1-11]
That was a very good observation. It is true that sunsets appear more colorful than sunrise. This is because throughout the course of the day, human activity (cars, pedestrian traffic, sound waves, etc.) -of which there is a lot more in the daytime than nighttime- kicks up a lot of dust particles, pollutants, etc. These particles can refract light causing the normally white light to be split (like the making of a rainbow) into many other colors. At night, human activity diminishes, and so the light during a sunrise is not as colorful.
Greg (Roberto Gregorius)
As I understand it, the chief difference is that the air is usually warmer during a sunset. This means more water vapor is present. Another possibility is that there is often a “ground inversion” of cold air at low altitudes at sunrise. These often contain fog, which may also cut down on the light reaching you.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
The sunrise and sunset would be symmetric if there were no atmospheric effects. My guess is on the moon it would be difficult to tell the difference. The redness of the sunset is due to a phenomenon called Rayleigh scattering (named after Lord Rayleigh who investigated the phenomenon). Light scatters from particles in the air depending on its wavelength and the size of particle in the air. Longer wavelengths, more reddish light, scatter less than shorter wavelengths, more bluish light. If we have white light, all wavelengths, traveling in a straight line to us from the sun, blue light will scatter in all directions leaving more red light to reach our eyes. The more particles in the air, dust, smog, etc, the reddish the light will be. At sunset, the sunlight travels through more atmosphere and will appear redder than when it is directly overhead. Rayleigh scattering also explains why mountains appear blue. The mountains are illuminated by the scattered light, most bluish, since those wavelengths scatter the most.
The likely change between sunset and sunrise comes from the different levels of atmospheric dust between morning and at night. More activity occurs during the day to load the atmosphere with more dust. Human activity will introduce scattering particles in the air. Sunlight also is a driving force for weather, and dust can be kicked up by winds. The perception that sunrise is gloomier may arise from the same cause—scattered light during sunset will give the perception that the surrounding scene is receiving more light, and “brighter” light since it will favor the blue end of the spectrum. Also, contrast may be sharper in sunrise since there is more direct light from the sun and less scattered light. Note the stark contrast and gloomy landscape of the moonscape when the astronauts were on the moon.
There may be other effects than Rayleigh scattering, but it is a definite contributor. The next time you see very red skies observe that there will be some consequent change in the atmosphere. Fires, dust storms, even volcanoes will deposit many particles in the air, and Rayleigh scattering will be very pronounced. An interesting way to test this explanation would be to measure the spectrum at sunrise and sunset and see how it changes. Scattering would shift the overall intensity of sunlight between sunset and sunrise, and change the relative intensity of light on the red versus the blue side of the color spectrum.
The one significant difference between sunset and sunrise is the quality of the atmosphere through which the sunlight passes. Sunrise light passes through an atmosphere that has had more darkness and sunset passes through an atmosphere that has experienced light. These conditions could create different light scattering properties. One might expect more dust and a higher water vapor content in the path of sunset light.
Jim Tokuhisa, Ph.D.
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Update: June 2012