WHY DO WE SAY THE SKY IS BLUE?

What color is the atmosphere? Actually the atmosphere, which we call the air, has no color. As you know, the sky isn't always blue. At night it looks black. At sunrise or sunset it might look red or orange. On sunny days the sky looks very deep blue and on days when there is a lot of moisture in the air, it looks gray. What makes the difference?

SCATTERING THE LIGHT

The air itself is transparent (meaning that we can see through it) and it doesn't have any color. But, when the sun shines on Earth's atmosphere the light is bent and scattered by many particles in the atmosphere. Because of a special way that light is designed, different colors of light bend differently when they go through a transparent object. For example, when light passes through a specially-shaped piece of glass we call a prism, the different colors of light are bent in different amounts. We can see the different colors of red, orange, yellow, green, blue, and violet. The same effect is what causes a rainbow when the sun shines on raindrops in the air. The colors of light passing through the tiny drops of water bend differently, so we can see that what we call white light is really made of of many colors of light. The light that is bent the most is the violet or blue light. The light that is bent the least is red. When light from the sun passes through the atmosphere it interacts with particles in the air causing it to bend and scatter. Red light scatters the least. Blue and violet light which bend the most are also scattered the most. Since our eyes are less sensitive to the violet color and with the blue light scattered throughout the sky, the sky looks blue. The light coming directly from the sun is not scattered so it contains all of the colors, including red, orange, and yellow. That's why the sun looks yellow compared to the blue sky.

THE RED SUNSET

The same scattering of light is the reason why the sky looks red at sunset. When the sun is close to the horizon, the light rays from the sun have to pass through more of the atmosphere to reach your eyes. Because the light has passed through more atmosphere, more of the blue/violet light has been scattered out and we see more red/orange light.

THE BLACK NIGHT SKY

At night, when light from the sun is not being scattered, we can see right through the atmosphere. The transparent nighttime sky allows the stars to be clearly visible. If you live in a city or close to a shopping center or some other area where there are a lot of electric lights, you may not be able to see the stars very well. The lights coming from the earth are scattered by the atmosphere and make the sky less transparent. For that reason it's harder to see the stars in an area where there are a lot of electric lights. We call this “light pollution.” Astronomers try to put their telescopes and observatories in locations where there are fewer lights so they can get a better view of the night sky. If you can go to a place in the country far away from street lights on a clear night you can enjoy the view of many, many stars in the sky.

THE THIN BLUE LAYER

On a day when the humidity is low (the air is dry) you may notice that the sky is a darker and deeper blue. That's because there are fewer droplets of moisture in the air to scatter the light. If you go to a high altitude such as the top of a mountain, you may find that the sky is a deeper blue there also. That's because as you go to a higher altitude there is less atmosphere between you and the sun. If you could keep going higher in a rocket, you would see the sky become darker and darker until finally the sky would be black because you have left the earth's atmosphere. You would be in what we call “space.” In space there is no atmosphere to scatter the light. The stars and the sun would be extremely bright lights and the rest of the sky would be black. While you were out there in space, if you could look back, you would see a blue haze around the Earth. That blue haze where the light is scattered is our atmosphere, the air that we breathe. It is what makes life possible on this planet. From space you would see that it's a very thin layer compared to the size of Earth. We can thank God that it's there and that we can enjoy that blue sky.