February 27, 2013

Green Sky or Blue Sky? Light Determines Sky Color

Q: Why is the sky blue and not some other color?

A:  From space, the Earth is seen totally surrounded by blackness, says Texas A&M University’s Brent McRoberts.  “But the blue sky and white clouds we see are the result of light being scattered in all directions,” he explains. “We know that light is coming from the sun, and this light contains all of the colors within the spectrum. The colors of light travel in waves of different lengths, and about 18 miles up, light encounters air molecules. These air molecules allow most of the light to pass through.  But blue and violet travel at small wavelengths which get scattered by these very small air molecules across the sky.  This is also known as Rayleigh scattering, which makes the sky look blue.  During a sunrise or sunset, the sunshine has to pass through more atmosphere, so the shorter wavelengths are almost completely scattered away, allowing a reddish color to be seen.”

a cloud in a deep blue sky

Scattered light determines sky color. Photo: Shutterstock

Q: So why does the sky look gray during a hazy day?

A:  On hazy days, large particles in the air make the sky appear gray or even white, McRoberts explains. “These larger particles tend to scatter more wavelengths of light in the color spectrum,” he says.  “Hazy air has a lot of water molecules, and these molecules can scatter light of all wavelengths, not just blue.  In heavily polluted air, the sky may appear yellow or brown, and this is due to the particles being able to scatter the light to produce these colors. This phenomenon is called Mie scattering. To sum it up, the way light is scattered determines the color of the sky.”

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Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.

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