Pink Mountains and Red Sunsets

Q: Why is the setting sun red? Why do mountains turn pink in the setting sun?

A: Any west-facing mountain turns golden, then pink as it reflects light from the setting sun. The sun becomes dimmer and redder as it sets and the mountain reflects this red light.

At sunset the red-orb sun hovers on the horizon, changed from the blinding-white disk we see at noon. Air, smoke, and dust cause this amazing transformation. When the sun is low, its light must go through more atmosphere (about 38 times more) than when it's overhead. A sunray beaming through thick atmosphere loses violet, blue, and green light on the way. Red light shines through relatively unhindered. That's the main reason why the white sun turns red at sunset.

Light is made up of all colors: some better at traveling straight to your eye than others. An incident sunbeam coming into our atmosphere may smash into an air molecule or particle of dust or smoke. If it does, its blue light is three times more likely than red to scatter. The sunbeam re-radiates blue light in all directions. Less of the original light gets through because of this scattering. The light that does get through is red.

Water and ozone enhance the reddening effect by absorbing violet, blue, and green light.

The beauty of the show transcends the mechanics. As the sun dips towards the horizon, the sunbeam path through the atmosphere gets longer and longer. And the sun changes from yellow to wondrous colors of peach, magenta, orange, pink, and finally red. The show changes daily with varying amounts of dust, smoke, and water in the atmosphere. Never the same watermelon pink from day to day. When the show's over, the deep blue shadow you see moving up the mountain is the Earth's shadow.

(Answered April 11, 2001; updated Aug. 30, 2007)

Further Reading:

Matt McIrvin: Why the sky is blue-dipole scattering

Science Made Simple: Why is the sky blue

Color and Light in Nature by D. Lynch and W. Livingston

USATODAY.com graphic: What gives the sky its color