Why is the sky blue?

It is easy to see that the sky is blue. Have you ever wondered why? A lot of other smart people have, too. And it took a long time to figure it out!

The light from the Sun looks white. But it is really made up of all the colors of the rainbow.

A prism separates white light into the colors of the rainbow.

A prism is a specially shaped crystal. When white light shines through a prism, the light is separated into all its colors.

If you visited The Land of the Magic Windows, you learned that the light you see is just one tiny bit of all the kinds of light energy beaming around the Universe--and around you!

Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves.

Different colors of light have different wavelengths.

All light travels in a straight line unless something gets in the way to--

* reflect it (like a mirror)

* bend it (like a prism)

* or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

Atmosphere scatters blue light more than other colors.

Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions. Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue.

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.

Red sky at sunset

Sometimes the whole western sky seems to glow. The sky appears red because larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows.

Red sun at sunset.
Why Does Scattering Matter?

How much of the Sun's light gets bounced around in Earth's atmosphere and how much gets reflected back into space? How much light gets soaked up by land and water, asphalt freeways and sunburned surfers? How much light do water and clouds reflect back into space? And why do we care?

Sunlight carries the energy that heats Earth and powers all life on Earth. Our climate is affected by how sunlight is scattered by forests, deserts, snow- and ice-covered surfaces, different types of clouds, smoke from forest fires, and other pollutants in the air.

excerpt taken from http://spaceplace.nasa.gov/en/kids/misrsky/misr_sky.shtml