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updated: 8/30/2011 8:52 AM

Color spectrum makes sky look green before tornado

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  • Storm clouds roll into Libertyville with high winds and heavy rain close behind.

      Storm clouds roll into Libertyville with high winds and heavy rain close behind.
    PAUL VALADE | Staff Photographer, 2010


You wanted to know

"Why does the sky turn green before or during a tornado?" asked Katherine Crawford's fifth-graders at West Oak Middle School in Mundelein.

"Red sky at night, sailor's delight" is an old saying that forecasts good weather. The colors we see in the sky -- the red sky of a clear evening sunset or the brilliant colors of a rainbow -- come from the electromagnetic spectrum.

The spectrum is energy that is radiated in a wavy pattern. The widest waves, invisible radio waves, are at the low end of the spectrum. As the spectrum progresses the waves become smaller, with the smallest waves being invisible, atom-sized gamma rays.

Visible light sits in the middle of the spectrum. The larger waves in the visible spectrum are the reds, pinks and oranges. The smaller visible waves are blue or violet. Blue waves are more quickly absorbed and radiated by particles in the atmosphere like gas and dust, creating a blue tint to the sky.

As the sun sinks to the horizon at dusk, it takes longer for light waves to travel through the atmosphere. The most visible portion of the light waves becomes the larger waves of red, pink and orange.

Green skies signal severe storms.

"Some people believe that the sky turns green before a tornado and some believe that it turns green before a hail storm," said Lt. Blake Hamilton, U.S. Air Force meteorologist. "Neither response is exactly correct."

Hamilton said the mix of colors from the spectrum results in the eerie green cast to the sky.

When the temperature rises and severe storms begin to brew later in the day, the red wavelengths from the setting sun mix with the blue sky wavelengths and combine to make the sky seem green.

"The thicker water-intensive clouds offer a better chance for the sky to create a greenish tint," he said.