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Ask the Weather Guys: How is tornado intensity determined?
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Ask the Weather Guys: How is tornado intensity determined?

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On a day when there's severe weather in your area, you may find yourself under a tornado watch or even a tornado warning. What's the difference?

Q: How is tornado intensity determined?

A: A deadly tornado outbreak took place Wednesday through Friday in the southern United States.

Tornadoes are classified based on the damage the tornado does, which enables us to estimate the wind speed of its rotating winds.

All tornadoes are assigned a single number from the Enhanced Fujita scale, abbreviated EF, according to the most intense damage caused by the storm.

This scale is based on the research of Ted Fujita and uses a set of 28 damage indicators to such structures as barns, schools and trees. The degree of damage to each one is used to determine the EF scale number of every tornado.

Severe Weather South

Piles of debris remain after a tornado touched down, killing several people and damaging multiple homes, Thursday in Ohatchee, Ala.

The breakdown, and example damage, of the Enhanced Fujita scale is:

EF0

  • (weak): 65 to 85 mph; peels surface off some roofs; some damage to gutters or siding; branches broken off trees.

EF1

  • (weak): 86 to 110 mph; roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors.

EF2

  • (strong): 111 to 135 mph; roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes destroyed.

EF3

  • (strong): 136 to 165 mph; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown.

EF4

  • (violent): 166 to 199 mph; well-constructed houses and whole frame houses leveled.

EF5

  • (violent): 200-230 mph; strong frame houses leveled and swept away; steel-reinforced concrete structures badly damaged.

We do not classify the strength of a tornado until experts assess the damage it did to the area.

No matter the classification, all tornadoes are dangerous. During last week’s tornado outbreak at least five fatalities have been confirmed due to an EF2 tornado with another death due to an EF4 tornado.

Photos: Scenes of destruction after tornadoes in South

"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.

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