Eye-catching sunrises and sunsets also seem to favor certain times of the year. In the middle latitudes and over the eastern half of the United States, for example, fall and winter generally produce the most spectacular low-sun hues. These and a few other twilight topics are explored on the next few pages. What dust and pollution don't do It is often written that dust and manmade pollution are responsible for colorful sunrises and sunsets. If, however, tropospheric dust and haze were responsible for brilliant sunsets, cities such as New York and Los Angeles or London and Mexico City would be celebrated for their twilight hues.
Clean air is, in fact, the main ingredient common to brightly colored sunrises and sunsets.
To understand why this is so, one need only recall how typical sky colors are produced. The familiar blue of the daytime sky is the result of the selective scattering of violet and blue light by air molecules. Particles are good Rayleigh scatterers when they are very small compared to the wavelength of the light. Ordinary sunlight is composed of a spectrum of colors that ranges from violet and blue at one end to orange and red on the other. The wavelength of this light ranges from .47 um for violet to .64 um for red.