Home >> Science & Nature >> Nature

The Levels of Tornadoes

Tornadoes are the world's most violent storms. Although they're smaller in area than the monstrous hurricanes that occur over tropical waters, tornadoes spin at a much higher speed, and are capable of causing incredible destruction. In the United States, which experiences more tornadoes than any other country, the National Weather Service uses the Enhanced Fujita (EF) scale to rank the destructive power of tornadoes. This metric derives from a classification originally developed by the meteorologist Dr. T. Theodore Fujita.

Enhanced Fujita Scale
- Officially adopted by the National Weather Service in 2007, the Enhanced Fujita scale incorporates more estimation factors than the original 1971 scale. Like the first version, the modern scale ranks tornadoes into six categories, based on estimated wind speeds in three-second gusts: EF0, 65-85 mph; EF1, 86-110 mph; EF2, 111-135 mph; EF3, 136-165 mph; EF4, 166-200 mph; and EF5, where winds exceed 200 mph.
Damage Indicators
- To develop the scale, meteorologists considered varying degrees of damage to 28 indicators, including manmade structures and trees. For example, tornado effects on hardwood trees comprise five levels of damage, from snapped-off limbs to fallen trunks stripped of bark. Additional degrees of damage apply to large shopping malls, including the smashing of windows and skylights, the removal of roofs and the complete obliteration of the building. Tabulating estimated wind speeds for these levels of damage helped analysts to refine the Enhanced Fujita scale, and gauge the destructive power of a given tornado.
Tornado Escalation
- Although meteorologists still don't fully understand the mechanics of tornado formation, the basic factors include highly unstable air masses, heavy vertical wind shear and frontal weather systems. Tornadoes often develop when rotating columns of air in so-called supercell thunderstorms shift vertically as a result of strong updrafts.
Strongest Tornadoes
- EF4 and EF5 tornadoes are the strongest known. They are quite rare -- accounting for only 1 percent of U.S. tornadoes -- but cause the majority of damage and loss of life. When the rotating funnel of a big tornado is enhanced by the forward motion of the storm, a multiple-vortex tornado may suddenly arise, with subvortices whipping in and around the center. In May 2011, a multiple-vortex EF5 tornado -- one of the deadliest in U.S. history -- wrought immense destruction in Joplin, Missouri.

Nature