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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 8 August, 2007 Next Week's Question
Air molecules scatter wavelengths to create different colors in light
Question
Why are there different colors to lightning?

Question
Visible light is a combination of wavelengths from short wavelengths (violet) through long wavelengths (red). When all the visible wavelengths are mixed together, the color of light appears white. If you diminish the strength of some of the wavelengths, the overall color of the object changes.

When white light passes through the Earth's atmosphere, the molecules that make up the air (nitrogen, oxygen, and argon) will scatter the wavelengths of visible light disproportionately. Shorter wavelengths are scattered more than longer wavelengths by a factor of roughly 3.5:1. This causes a beam of white light to appear yellowish. As the light travels through more air, the shorter wavelengths are depleted even more and now the yellow wavelengths are being significantly depleted causing the light to appear reddish. So the more air between you and the light source the redder the object appears.

A lightning flash reaches temperatures of about 30,000 K (54,0000°F) which is five times hotter than the surface of the sun. Objects with this temperature would emit a brilliant white light. The closer the lightning flash is to you, the whiter it appears. Distant lightning flashes will have a yellowish color because the light has to travel through more air molecules. If there are raindrops between you and the lightning flash, the color will appear reddish. Sometimes fine dust particles are eroded from the surface by the wind beneath the cloud and lifted into the air. These particles will also scatter the light and give the lightning flash a yellow/orange appearance.

There are special forms of lighting which do not involve a flash but represent a continuous glow. Conditions necessary for this type of lightning require the base of a thunderstorm cloud to be close to the surface of the Earth, thus creating a large surface charge density. The result is a continuous electric spark called a glow discharge. This is similar to what happens inside a fluorescent tube. If you have a tall pointed object such as a chimney, telephone pole, or a roof pinnacle nearby, you will observe a violet/blue glow surrounding the object. The color comes from the excitation of the nitrogen and oxygen molecules in our atmosphere. This type of lightning would occur frequently during thunderstorms at sea and would appear as a violet/bluish glow on the mast of sailing ships and is referred to as St Elmo's fire.

Please remember that all forms of lightning are deadly. When a thunderstorm approaches your area, you should seek shelter inside immediately.

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