Why is Sunset Red?

The sun gives off the full spectrum (rainbow) of wavelengths of light, from the longest (red) to the shortest (violet).  (We will ignore the infrared and ultraviolet also given off, since these are not visible to our eyes.)  Together, a pretty even mixture of these wavelengths appears as "white light."  Most of these light waves travel through air pretty much unchanged, but those near the violet end of the spectrum, because their short waves are similar in size to air molecules, are scattered in all directions.  That is why a clear daytime sky is blue: that blue glow is short-wave light scattered off air molecules.  As the earth rotates and causes the sun to approach the horizon, the direct light will appear reddish, since more blue light has been scattered out of it.

Now consider a sun setting behind a few clouds.  As the sun drops, the cloud tops, shining by indirect, scattered light, appear blue.  Meanwhile--at a critical moment--the bottoms of the clouds, where they get the direct light "left over" from scattering, reflect this reddened light to your eyes.  Together this color combination can make breath-taking sunsets. 

This limited time adds to the value of a sunset: the time when the angle between the sun and the earth at your location is allows the sunlight to sneak between clouds and ground lasts only a moment.  If, during a sunset you watch the sky to the east, you can often see where this moment has already past, and the clouds have gone blue, while in the east that moment is still to come.

I don't see sunsets often, because of the trees west of my home.  But I caught this sunset last evening as I was coming out of a store.  I actually regretted my impulse to get out my camera: the most glorious moment passed while I was fumbling with it, and the photo does not come close to capturing it.  I should have skipped the camera, and captured it in my memory.  Besides being "quicker on the draw" than a camera, the eye and mind can focus in on what's important and ignore distractions such as steet lights,  trees and power lines.

a nice simple source of info is: http://spaceplace.nasa.gov/blue-sky/en/