There is a mean randomness to tornadoes that continues to puzzle storm researchers. Why do they form out of one severe thunderstorm but not another? On one day but not the next? Why is one more powerful than another? And most particularly, perhaps — why is one tornado 200 feet wide and another a mile or more in diameter?

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Forecasters and researchers can't really answer these questions. They can tell you the basic ingredients for a tornado outbreak — the collision of cold dry air aloft with warm moist air at the surface — but they can't say if the tornadoes are going to be narrow or wide or weak or powerful.

"This is one of the great mysteries," observes meteorologist Greg Carbin at the National Weather Service's Storm Prediction Center in Norman, Okla. Carbin recalls a 2003 outbreak when a rash of intense tornadoes 50-75 yards wide was followed by a monster with a girth of a mile and a half wide that totally wrecked Greensburg, Kan. "How can nature do this?"

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As Earth sciences go, the subject of weather is young, and the science of tornadoes is a newborn babe. Until 1950, in fact, government forecasters couldn't even mention the word "tornado" in a forecast for fear of sowing public panic with a rash of false alarms.

Even now, unlike other weather phenomena, basic information about wind speed inside a tornado is not measured instrumentally but rather is estimated by the extent of the damage it does. This is weather at the edge of chaos — forces so powerful that most instruments can't survive them.

The widest tornado ever observed — 2.5 miles in diameter — struck May 22, 2004, in Hallam, Neb. A recent study draws a statistical link between tornado size and damage, but that's at least partly because a bigger, wider storm covers more ground.

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Carbin notes that the width of a tornado has "something to do with intensity and the environment of the updraft," but researchers can't yet say exactly how these conditions work together to alter the size and shape of a tornado or the intensity of its winds. "Vortices in the atmosphere can range in size from something very, very small to something very, very large," says Carbin.

Among all the mysteries of tornadoes, cruelest is the serendipity of their location — exactly where they strike. "Yesterday in Joplin, a very intense tornado moved through a populated area," Carbin notes. "If it had happened just a few miles to the south, we probably wouldn't be talking about it."

IMAGE: Storm debris from a tornado that struck May 22, 2011, litters whole neighborhoods in Joplin, Mo. The mile-wide tornado ripped through the densely populated town of Joplin, killing at least 116 people. (Photo by Julie Denesha/Getty Images)