Quote:
Originally Posted by speeder
Why does the sky turn green?
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The color is reflected up into the sky from the lost city of Oz, which of course
is in another dimension and is not visible directly, but its green glow is seen
as reflected light on water droplets and dust particles in the atmosphere,
much like moonglow.
But seriously...
Theory #1. Because air molecules scatter light. To understand this first theory, try picturing mountains in the distance, which often look a deepening blue. That blue color stems from the scattering of light by air molecules. The same reason, scattered light, causes the sky to look blue. Now remember that the strongest storms tend to happen late in the day. Some experts think that, before a thunderstorm, golden-reddish light from a sun low in the sky – and a natural bluing effect of the air – combine to create a green sky. The storm provides a dark backdrop and offsets this greenish or yellowish hue.
Theory #2. Because water is intrinsically blue. A different meteorological theory holds that storm clouds themselves may help make the color of the sky bluish-green. To understand this theory, you first need to know that water is intrinsically blue (sometimes you can see the bluish tint of water in a white bathtub). It could be that the storm clouds — which are filled with water — provide the color blue, which, again, is illuminated by the golden light of a low sun to create the color green.
To sum up, the reason for green skies before a storm isn’t entirely known. But it is known that a greenish-yellow sky before a storm is common in some parts of the world, while totally absent in others. More:
Why does the sky change color before a tornado? | Earth | EarthSky
Also: The first question researchers faced: Is a green sky real, or just an optical illusion caused by light reflected off the ground and back up into the sky, as some green sky dissenters suggest? Frank Gallagher, now a meteorologist for the U.S. Army at Dugway Proving Ground in Utah, tackled this issue for his thesis at the University of Oklahoma. He joined a tornado-chasing research team called VORTEX and recorded the wavelengths of light coming from storms in Texas and Oklahoma using a spectrophotometer, a tool about the size of an old video camera that can measure the color and intensity of light.
Gallagher found that the dominant wavelength of light was green in several severe thunderstorms and that the color was independent of the terrain underneath the storm. As meteorology professor William Beasley, Gallagher's advisor at Okalahoma, put it, "[He] measured green wavelengths of light over a green wheat field and over freshly plowed fields with red-brown Oklahoma dirt."
Threatening green skies during a thunderstorm also proved entirely independent of the type of severe weather that came with it. Gallagher measured hailstorms where the dominant wavelength of light was green as well as hailstorms where it was the typical gray-blue color of thunderstorms. Tornado-producing storms proved similarly divorced from any particular sky color, other than dark.
Researchers remain undecided about the exact mechanisms that cause the sky to appear green in certain thunderstorms, but most point to the liquid water content in the air. The moisture particles are so small that they can bend the light and alter its appearance to the observer. These water droplets absorb red light, making the scattered light appear blue. If this blue scattered light is set against an environment heavy in red light—during sunset for instance—and a dark gray thunderstorm cloud, the net effect can make the sky appear faintly green. In fact, green thunderstorms are most commonly reported in the late afternoon and evening, according to Beasley.
More:
Fact or Fiction?: If the Sky Is Green, Run for Cover—A Tornado Is Coming - Scientific American