WRAL WeatherCenter Blog

Yesterday (8/2) when the sun was setting it was blood red. What causes this? By the way, it was absolutely gorgeous!

Posted August 5, 2007

MIKE MOSS SAYS:      Millie,       The sun (and moon) take on orange or reddidh hues when they descend toward the horizon because of the effects of differential scattering of light waves by air molecules and by tiny dust, smoke and other pollutant particles in the air. These molecules and particles, when significantly smaller than the wavelenght of light, scatter the shorter wavelengths of light (blue, indigo and violet) to a greater degree than longer wavelengths like yellow, orange and red (a process called Rayleigh scattering). When the sun is high in the sky, we see the blue-dominated light scattered indirectly toward us other parts of the sky, while the sun itself appears a bit yellow. As the sun sets, more and more of the shorter wavelengths are scattered off to the side, because the sunlight grazing the horizon is passing through much more air than sunlight coming from a high angle. This leaves mainly the orange and red wavelengths for us to see, and makes the sun look orange or red. In a very clean atmosphere, the sun may still have a yellow-orange color even fairly close to the horizon. However, if there are lots of tiny smoke particles or other pollutants in addition to the air, the scattering effect is even stronger and the sun may take on a very dark red appearance as even some of the orange light is scattered away, leaving red wavelengths predominant.


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