If the sky turns green during a thunderstorm, gather up your pets and other loved ones and head for the cellar, a twister is on the way. So goes the common wisdom in much of the central U.S.—and other tornado-prone regions in the world, like Australia—when faced with a threatening sky (although some swear green means hail). Scientifically speaking, however, little evidence supports either the tornado or hail claims, though there is some evidence for green thunderstorms.

Over the past 15 years, a small group of scientists have weathered the elements working on green thunderstorms as a pet project, publishing a handful of articles in meteorological journals. All point to the existence of green skies with severe thunderstorms but no direct connection to tornadoes or hail can be made.

"Green skies are associated with severe weather," says physicist and occasional green thunderstorm guru Craig Bohren at Pennsylvania State University. "In areas where tornados are common, they are said to be the cause of green storms. Or you will be told, often with considerable vehemence, that hail causes the greenness. Both explanations are easily refuted by observations."

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.

In a paper published in the Journal of Applied Meteorology, Gallagher also suggested that green thunderstorms might occur more frequently than thought. Because it gets quite dark during thunderstorms, the purity of light may be too poor for observers to see the color on most occasions.

Other research on green thunderstorms is limited and not well funded. As Penn State's Bohren says, this is "not exactly a hot topic of research. Indeed, being curious about them can be hazardous to one's career." For example, his small grant from the National Science Foundation for the portable spectrophotometer Gallagher used was derided by then Speaker of the House Newt Gingrich's office and Richard Pombo, then a Republican congressman from California, who denounced Bohren in the Congressional Record. (Of course, neither politician hailed from "tornado alley.")

Joshua Wurman at the Center for Severe Weather Research in Boulder, Colo., agrees: "I wouldn't call [the green sky] one of the burning issues in tornado science." Nevertheless, the research that is out there suggests that the green colored sky some might observe during a severe thunderstorm does have limited predictive power for those who live in areas where severe weather occurs, like Dorothy—and her little dog, too.