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.")
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4 Comments
Add CommentI moved to the mid-west, late in life and was surprised to see the eerie green cast sky. Unlike a blue sky, the air around you, when say looking at the house across the street, takes on a strange glow that almost makes you squint; it goes from gray darkness to a bright radium-dust effect like someone turned on a florescent bulb. I've also seen that happen where the glow is more orange than green. The green sky, as they said, does not always indicate that a storm will hit where you are, let alone a tornado (something I haven't seen here). I did get to see something that I'd always wanted to see and fascinated me as a child (from Portland, Oregon): Hail the size of golf balls. That was preceded by a green sky and followed by a clear sunny sky. The hail was lenticular but held up one way, the profile on most that fell was the easily size of a golf ball. Be careful what you wish for as a child. :-)
Reply | Report Abuse | Link to thisWell, I suppose that where you live might make a difference in how well these various tips and old wives' tales work, but I will say two things. 1. if you are in Dallas, Texas and see a green sky you can almost always expect hail. and 2. if you are in Wichita Falls, Texas and a storm front is coming through...For the love of G_D, Man... GO SOMEWHERE ELSE!!
Reply | Report Abuse | Link to thisI've lived on the prairies in both the US and Canada for 65 years. If the sky turns green, TAKE COVER! It may not be a tornado, but you can be sure the weather is about to take a turn for the much worse!
Reply | Report Abuse | Link to thisI have experienced this just once. The sky was a very sickly unnatural green and the air itself was very weird. Yes, that is not exactly a great description, but I have no idea how to put it into words. I never saw the tornado, but saw on the news that it had touched down less than a mile from where I was at the time.
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