Why Are Sunsets and Sunrises Red?

Rayleigh Scattering From Atmospheric Particles

Oct 7, 2007 Paul A. Heckert

Small particles in the atmosphere scattering light cause both sunsets and sunrises to be red.

Red Sunsets and Sunrises

We have all, at some time or another, enjoyed the beauty of a spectacular red sunrise or sunset. While watching nature's show many people will wonder what causes the sun to be so red when it is just above the horizon. Why are sunrises and sunsets red?

Particles in Earth's atmosphere cause this effect.

Rayleigh Scattering

When light bounces, or reflects, off small particles, it is called scattering. Rayleigh scattering is the type of scattering that occurs when the particles are smaller than the wavelength of the light being scattered. In 1871, Lord Rayleigh worked out the mathematical law that describes how Rayleigh scattering works.

Rayleigh found that shorter wavelength light is scattered much more efficiently than light at longer wavelengths. Mathematically, the amount of scattering is related to the fourth power of the wavelength. So blue light with a wavelength of about 400 nanometers is scattered about 10 times more efficiently than red light with a wavelength of about 700 nanometers.

Earth's atmosphere contains lots of oxygen and nitrogen molecules which are smaller than the wavelength of light. It also contains lots of dust particles. They are however usually larger than the wavelength of light so the Rayleigh scattering law does not apply to the dust particles.

Why Are Sunsets and Sunrises Red?

White light coming from the Sun contains all colors of light from red to blue. The molecules in Earth's atmosphere do not scatter much of the red light, but they do scatter a significant amount of blue light. This effect causes the blue sky.

These molecules scattering light also cause the Sun to appear redder than it really is. When the Sun is high in the sky the amount of reddening is small. However the Sun will still appear redder from the ground than from space because the atmosphere scatters some of the blue light.

When the Sun is low in the sky, this effect from Rayleigh scattering is much greater. Much more of the blue light coming from the Sun is scattered away from the direct path towards our eyes. Hence the Sun will appear very red when it is low in the sky.

The same thing happens to the Moon. It often appears very red when it is low in the sky for the same reason.

Much of the beauty of red sunsets or sunrises comes not from the red Sun but from the red clouds just above the western or eastern horizon. Red sunlight reflecting off these clouds gives them the same rich red color the Sun has near the horizon. So in a spectacular sunset or sunrise the entire horizon is red.

The effect can increase when the Sun sets or rises over a large city. Pollution particles increase the effect by scattering or absorbing more blue light than red light.

Enjoy your next spectacular red sunrise or sunset and remember that Rayleigh scattering off molecules in Earth's atmosphere causes the effect.

Further Reading

Knight, R., Physics for Scientists and Engineers, Pearson, 2004.

The copyright of the article Why Are Sunsets and Sunrises Red? in Physics is owned by Paul A. Heckert. Permission to republish Why Are Sunsets and Sunrises Red? in print or online must be granted by the author in writing.