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Damage control: Tornado scale revised

The National Weather Service has for the first time changed its tornado scale after learning that much weaker winds than previously thought can destroy homes and turn cars into missiles.
/ Source: The Associated Press

The government has for the first time changed its system of categorizing tornadoes after learning that much weaker winds than previously thought can create the most powerful funnel clouds that disintegrate homes and turn cars into missiles.

The National Weather Service said Thursday it had made changes to the Fujita Scale, a three-decade-old system of ranking a tornado’s strength, to align wind speeds more closely with actual damage.

“It was apparent that many of the speeds used in the estimates were too large, said Joe Schaefer, director of the service’s Storm Prediction Center. “The scale guiding wind speeds wasn’t in tune with reality.”

The change was introduced at the American Meteorological Society meeting in Atlanta. However, the new system will not fully go into effect until February 2007, giving weather scientists time to adjust to it.

Under the old system, created in 1971, an F-5 tornado — considered the most powerful of tornadoes — was capable of destroying a typical frame house, with wind speeds estimated at 261 mph to 318 mph. Since then, engineering studies have shown that much slower winds could cause the same damage.

“It doesn’t take 300 mph winds to totally destroy an ordinary frame house,” said Greg Forbes, a former member of Penn State’s meteorology department who studied tornadoes under Theodore Fujita, the University of Chicago professor who created the scale. Forbes now works for The Weather Channel.

Under the new system, an F-5 tornado — which can disintegrate a strong frame house after lifting it off its foundation or badly damage reinforced concrete buildings — has wind speeds of at least 200 mph.

Because the new system still uses actual tornado damage to estimate wind speeds, officials said it is not likely that the new system’s lower wind speed rating for the F-5 tornado will result in more tornadoes being classified with the nation’s top tornado rating.

The old system rated tornadoes only based on damage to homes. The new system classifies tornadoes based on damage to 18 other types of structures, including trees, mobile homes and other types of buildings.

“If a tornado went over a row of trees and didn’t hit a house, there was no way to estimate the scale,” Schaefer said.