Why Is the Sky Blue?

Like most curious peopleyou have probably asked at some time“Why is the sky blue?” Or if you saw a beautiful sunset or sunriseyou might have asked“Why is the sky red?”

It’s so obvious that the sky is blueyou might think the reasons would be just as obvious. They aren’t! Of all the colors of the rainbowwhy blue?

Couldn’t the sky just as easily be green? Or yellow? When we see a rainbowwe do see green and yellow in the skyas well as bluevioletorangeyellowredand everything in between.

The white light coming from the Sun is really made up of all the colors of the rainbow. We see all those colors when we look at rainbows. Raindrops act as tiny prisms when lit by the Sunbending light and separating it into its different colors.

But why are there different colors? 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 oceanlight energy travels in wavestoo. What makes one kind of light different from others is its wavelength - or range of wavelengths. Visible light includes the wavelengths our eyes can see. The longest wavelengths we can see look red to us. The shortest wavelengths we can see look blue or violet.

The wavelengths in this picture are not to scale. A red light wave is about 750 nanometerswhile a blue or violet wave is about 400 nanometers. A nanometer is one-billionth of a meter. A human hair is about 50,000 nanometers thick! So these visible light wavelengths are veryvery tiny.

Another important thing to know about light is that it 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)

As the white light from the Sun enters Earth’s atmospheremuch of the redyellowand green wavelengths of light (mixed together and still nearly white) pass straight through the atmosphere to our eyes. The blue and violet waveshoweverare just the right size to hit and bounce off of the molecules of gas in the atmosphere. This causes the blue and violet waves to be separated from the rest of the light and become scattered in every direction for all to see. The other wavelengths stick together as a groupand therefore remain white.

So what happens to all the “non-blue” wavelengths? They are still mixed togetherunscattered by the atmosphereso they still appear white. The scattered violet and blue light dominates the skymaking it appear blue. What happens to the violet? Some of the violet light is absorbed by the upper atmosphere. Alsoour eyes are not as sensitive to violet as they are to blue.

Closer to the horizonthe sky fades to a lighter blue or white. The sunlight reaching us from the horizon has passed through even more air than the sunlight reaching us from overhead. The molecules of gas have rescattered the blue light in so many directions so many times that less blue light reaches us.

What Makes a Sunset red?

As the Sun gets lower in the skyits light passes through more of the atmosphere to reach you. Even more of the blue and violet light is scatteredallowing the reds and yellows to pass straight through to your eyes without all that competition from the blues.

Alsolarger particles of dustpollutionand water vapor in the atmosphere reflect and scatter more of the reds and yellowssometimes making the whole western sky glow red.

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 waterasphalt 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 scatteredreflected back to spaceor absorbed by forestsdesertssnow- and ice-covered surfacesdifferent types of cloudssmoke from forest firesand other pollutants in the air.

Just as Earth's atmosphere bends and scatters light that passes through it from the Sun to the surfacethe atmosphere affects light reflecting off the surface back into space.

That is why satellites can perform what is called remote sensing from space and reveal a great deal about the surface and about the atmosphere. Instruments on satellites such as the Geostationary Operational Environmental Satellites-R (GOES-R) Seriescan measure the intensity of light of different wavelengths. Analyzing that informationscientists find out surface and atmospheric temperatureslevels of carbon dioxidewater vaporpollutantsozoneand other trace gases.