Have you ever wondered why there is a red sky during sunrises and sunsets?
To understand why this is so, one need only recall how typical sky colors are produced.
Sunlight passes through the atmosphere before it reaches us. The familiar blue of the daytime sky is the result of the selective scattering of sunlight by air molecules.
Scattering is the scientific term used to describe the reflection or re-direction of light by small particles.
We also know that sun light is composed of seven colors — Violet, Indigo, Blue, Green, Yellow, Orange and Red. (VIBGYOR). These have different wavelengths.
Air molecules are much smaller than the wavelength of these colours. Thus, air is a good scatterer. But because air molecules are slightly closer in size to the wavelength of violet light than to that of red light, pure air scatters violet light three to four times more effectively than it does the longer wavelengths.
In fact, were it not for the fact that human eyes are more sensitive to blue light than to violet, the clear daytime sky would appear violet instead of blue!
In the mornings and evenings, when the sun is near the horizon, the rays have to travel about fifty time’s longer path in the atmosphere to reach us than it does in the noon.
Because this lengthened path results in an increased amount of violet and blue light being scattered out of the beam by the nearly infinite number of scattering "events" that occur along the way, the light that reaches an observer early or late in the day is noticeably reddened.
It is often written that natural and manmade dust and pollution cause colorful sunrises and sunsets. If that were the case, cities like New York, Beijing, Tokyo, California, and closer home Delhi, Mumbai etc would have had more beautiful sunsets than the Saharan deserts. The truth is is that tropospheric aerosols – when present in abundance in the lower atmosphere as they often are over urban and continental areas, actually play a part in subduing the colors and add onto the haze.
Clean air thus is the main ingredient common to brightly colored sunrises and sunsets.