The government will use a new classification system to rate the power of tornadoes, so that estimated wind speeds are more closely aligned with actual damage.
The National Weather Service announced the change at the American Meteorological Society meeting in Atlanta. However, the new classification system will not go into effect until February 2007. Under the old system, created in 1971, an F-Five tornado -- considered the most powerful of tornadoes -- was capable of destroying a typical frame house, with wind speeds estimated at 261 to 318 miles an hour. Since then, engineering studies have shown that much lower-speed winds could cause the same damage.
Under the new system, an F-Five tornado has wind speeds of at least 200 miles an hour. 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.