What makes a red sunset?

As the sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

Sometimes the whole western sky seems to glow. The sky appears red because small particles of dust, pollution, or other aerosols also scatter blue light, leaving more purely red and yellow light to go through the atmosphere.

At sunrise and sunset, the Sun is very low in the sky, which means that the sunlight we see has traveled through a much thicker amount of atmosphere. Because blue light is scattered more strongly by the atmosphere, it tends to be scattered several times and deflected away in other directions before it gets to us. This means that there is relatively more yellow and red light left for us to see.

The diagram below shows that at midday, light has less atmosphere to travel through. Reaching Earth's atmosphere, it is scattered in all directions, therefore we see blue sunlight. Blue is scattered more than other colors because it travels as shorter, smaller waves. However, at sunset, light has further to travel through the atmosphere. The shorter wavelength blue light is scattered further, as the sunlight passes over a greater distance, and we see the longer wavelength yellow and red light. These effects are a cause of Rayleigh Scattering.   

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