Meteorologists have long used infrastructure damage to calculate tornado wind speeds, but that metric underestimates the real number of truly terrifying storms by as much as a quarter, a new study suggests.
According to the U.S. National Weather Service, monster tornadoes “are exceedingly rare, accounting for only about 1% of all twisters,” writes The Washington Post. “But that reassuringly small figure may be a product of the imperfect way that experts gauge tornado intensity.”
That’s the conclusion of a study recently published in the Proceedings of the National Academy of Sciences. The research found the incidence of mega-twisters—those ranking an EF-4 or EF-5 on the Enhanced Fujita scale of tornado intensity—to be “upward of 20%” of all supercell tornadoes.
The yawning disparity in the estimates comes down to process: while the Weather Service has long relied on “the damage tornadoes leave in their wake” as a categorization method, the University of Illinois researchers used radar data collected up close and personal, and in real time.
That the NWS has managed to lowball the incidence of the most severe tornadoes so severely owes to its dependence on examining wreckage after the fact: such a methodology works well where tornadoes touch down around a lot of buildings or trees, but underestimates any storms that move over a rural prairie grassland, leaving little trace of its passage.
“If a tornado goes through open fields, it essentially gets rated as a zero or a one,” said study lead author Joshua Wurman, an adjunct professor at the University of Illinois Department of Atmospheric Sciences. He said that rating would be applied even to a tornado with winds approaching 500 kilometres per hour.
Going a long way toward correcting this oversight, Wurman and fellow U of Illinois atmospheric scientist Karen Kosiba used a mobile radar system, designed by Wurman in 1995, that they call “Doppler on Wheels”. The system “allows experts to capture high-resolution radar cross-sections of live tornadoes from about a mile away.”
Analyzing data gathered from 120 tornadoes over the Great Plains between 1995 and 2006, the researchers found that “between 20 and 25% of supercell tornadoes boast wind speeds that would cause EF-4 or EF-5 levels of damage, even though only about 1% of tornadoes are typically rated EF-4 or EF-5.”
They concluded that, “on average, tornado ratings are 1.2 to 1.5 categories too low.”
The Post says the Doppler on Wheels radar study will now “be used by a joint committee of the American Meteorological Society and the American Society of Civil Engineers, which is working to improve tornado wind speed estimates made by the National Weather Service” by refining the Enhanced Fujita scale. The updated tool and improved data can then be used to improve the way homes are built in tornado-prone areas.