Tornadoes in Wisconsin, Montana spur early warning system
The VORTEX-2 tornado research project completed last week aims to increase warning times and reduce false alarms.
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This year alone, tornadoes have appeared as far outside of tornado alley as California, Washington, the Carolinas, and the Bahamas. Earlier this month, preliminary reports indicate tornadoes touched down in northern Vermont, New Hampshire, and Maine.Skip to next paragraph
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"Tornadic activity takes place in far more places than many people realize," says Anthony Del Genio, a researcher at the National Aeronautics and Space Administration's Goddard Institute for Space Studies (GISS) in New York.
Indeed, while the United States carries the dubious distinction of being the tornado capital of the world – with tornado alley as the Capitol rotunda – researchers have identified a range of tornadically active regions in the world. Southern Africa, where some researchers argue the most intense thunderstorms on the planet occur, is on the list, as are Australia, China, and portions of southern Argentina.
At least two major ingredients are needed to form the powerful "supercell" thunderstorms that spawn tornadoes: warm, moist air at the surface, and a dramatic change in wind speed or direction at higher altitudes.
As the warm air rises into colder air masses, its moisture condenses into water droplets and ice crystals that form towering thunderheads.
Meanwhile the difference in wind direction or speed sets up a rolling mass of air at the boundary between the moving layers – a bit like rolling a paper-towel tube between two hands.
If the rising air from the building storm cloud moves through this vortex, the updraft in effect stands the vortex on its end, imparting rotation to the whole storm cloud.
That sets up the environment in which tornadoes can form.
From there, things get tricky. One of the enduring questions scientists have been trying to answer is why some of these severe thunderstorms generate tornadoes and others don't.
One of the lessons from the VORTEX 1 tornado study, which took place 15 years ago, was that the problem of understanding tornado formation "was a lot harder than we thought" going into the project, says Wurman, who also participated in VORTEX 1.
He points to storm-cloud rotation as an example. "There were a lot of supercells that had rotation at mid levels and even low levels, but a lot of them still didn't make tornadoes."
On the other hand, the researchers encountered what might be called the Goldilocks benchmark for the kind of cloud temperatures that encourage tornado formation.