why is the sky blue

Everyday people all over the world go outside and look up at the exact same blue sky. It is an incredible thing that connects people everywhere. Its color is distinct and has puzzled and delighted people for ages. Why is the sky blue? People wonder. Why not orange or green or purple? Why not striped or plaid or spotted? It is a question both children and adults contemplate on a daily basis.

The skies enormous blue is a mystery that is fun to wonder at. But if you can believe it, there is a reason. Scientists have been studying this phenomenon of why the sky is blue for many years. Many different people have contributed questions, answers and research to help us know more about why the sky is blue. It has to do with the molecules in the atmosphere and light.

What do I need to know about the atmosphere?

The atmosphere is the air we live and breathe as well as the sky above us and the air all the way out to space. Anything in the earth is made up of tiny pieces of matter called molecules. Even you and I are made up of molecules. The atmosphere is made up of a combination of these molecules:

• Nitrogen (a gas) makes up 78% of the atmosphere.
• Oxygen (another gas, which we breathe) makes up 21% of the atmosphere.
• Argon (a gas), water vapor and ice make up less than 1% of the atmosphere.

Depending on where you are and what the weather is like, the specific amounts of all of these things change. For instance, the amount of water in the air will vary greatly depending on if you are on a boat in the ocean or in the middle of the desert. Certain places, such as near volcanoes, will have a higher concentration of dust in the air. Cities and urban centers are known to release other pollutant molecules and substances in to the atmosphere, changing the composition.

Nearest Earth, the atmosphere is the most concentrated with molecules. As you go higher and higher into the sky, the air thins out and eventually turns into space. The sky does not end abruptly—it is a gradual decrease of molecules of air and gas until you get to space, where there is none.

What do I need to know about light?

To begin with, light is an energy that travels through the sky in waves. This is called radiation. The light we can see, visible light is a part of an entire continuum of light called the electromagnetic spectrum. Some light in the electromagnetic spectrum we can see and some of it we cannot.

White light, from the sun or a light bulb, is not actually white but contains an entire rainbow of colors which blend continuously. This color continuum was discovered by physicist Isaac Newton in 1672 who used a prism to show the different colors in white light. All the colors have different energy, lengths and speed and size of waves.

Light will travel forever until it is disturbed. When it goes through the atmosphere and sky it bumps into dust and gas molecules. Depending on the wave size of the light and the size of the thing it has bumped into, the white light may stay intact or split and appear as certain colors.

• If light hits dust, it remains white (because dust is bigger than light waves).
• If light hits water vapor, it remains white (because water vapor is bigger than light waves).
• If light hits gas molecules, some of the colors are absorbed and released from the molecule in a different direction (because gas molecules are smaller than light waves).

Different colors within white light get absorbed (and released) in different capacities by gas molecules. For instance, reds do not get absorbed as often as blues. This absorption and release processes is called Rayleigh scattering.

Why is the sky blue?

The absorption and release phenomenon of Rayleigh scattering helps us answer this question. When light is moving through the atmosphere, most of the wavelengths pass through without being absorbed by molecules. Air does not affect red, orange or yellow light very much.

The air looks blue, because many blue molecules are absorbed by gas molecules. They do not just pass through, they stay in the air, are scattered all around and make the sky blue!

Next time you look up at the blue sky, remembering how molecules and light work will make the blue even more astounding.