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The Color of the Sky

Use Digital Cameras to Study the Color of the Sky

The Sun produces white light. White is the combination of all of the colors of the rainbow. When we view sunlight through a prism, we bend the colors so that they are separated. When sunlight shines through our atmosphere, it is bent and scattered by particles. Our atmosphere, because it is largely made up of nitrogen and oxygen, is most efficient at scattering blue wavelengths of light. If you look back at your data from the Color Investigations, you will see that if we remove blue from our white light, the color that we are left seeing is yellow. The scattered blue light leaves the sun looking yellowish.

But we all know that the sky is not always blue. At sunset and sunrise, the sky includes many more colors from throughout the spectrum. You may have picked up on the use of the word “efficient” above. Other colors of light are scattered by our atmosphere, but not as efficiently as the blue. When the Sun is high in the sky, its light has a shorter distance to travel through the atmosphere and the blue light is scattered. When the sun is low in the sky, the path is much longer and more colors of light are scattered.

While the question “why is the sky blue?” is one of general interest, the reason we are discussing it here is to start thinking about the effect that our atmosphere has on the light that travels through it. If we are going to take an image from space of light that shines from the Sun through the atmosphere and is reflected back up to space, then we must pay attention to what the atmosphere does to that light on its travels.

Materials

  • Digital camera to take pictures of the sky, or use the many sky photos available at the PicturePost website (go to the "Looking Up" folder for each post).

  • DigitalImageBasics software allows you to separate the layers of color in a digital photograph. Download the software.

What To Do...

  1. Open digital photos of the sky in the DigitalImageBasics software.

  2. Move the cursor into the image and notice the color of the highlighted pixel is displayed.

  3. Turn layers of color on and off to study the colors across the image more efficiently.

Question: Which of the three-color components (red, green, or blue) is dominant in the sky? Which color contributes the least intensity to the sky? See the contribution of a color by setting all of the computer display colors to a single picture color.

Question: If the sky is blue because of tiny atmospheric particles scattering blue light, why is the Sun yellow? Hint: this is a good time to take a look back at the Tri-Color Reference Chart that you created in Color Investigations.

Question: If the Sun appears to be yellow, why do clouds appear so white in the image? If you were riding on a satellite thousands of miles above the Earth, what would be the color of the “sky” (or outer space) and what would be the color of sunlight?

Extensions

    Study sky color over the course of a day or a week and note the change in the proportions of red, green, and blue light. Relate this to humidity and air pollution. For air pollution data, see the AirNow website.

    Use colored filters, simiilar to the gels used in the plant stress detection filters, and place over the camera lens and photograph the sky (see example below). For ordering filters, see the sources for equipment page.


Photo of sky without a color filter
Photo of sky without a filter.
Photo of sky with yellow filter (blocks blue light).
Photo of sky with yellow filter

See Forrest Mimms calibrated haze sensor that is based on measuring the color of the sky.

Back to Digital Image Investigations

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This is a picture of the sky using a camera with a fish-eye lens, allowing most of the sky to be seen at once. The color photograph is  followed by the red, green,  and blue layers of the same photograph.
Picture of the sky using a fish-eye lens
Picture is courtesy of Chuck Wilcox at the Museum of Science, Boston.
Sky with fish-eye lens but only displaying red

Sky with fish-eye lens but only displaying green

Sky with fish-eye lens but only displaying blue