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Tornado Factbox

   VOA News  May 23, 2011

   Tornadoes at a Glance

   Formation: Rotating thunderstorms or supercells cause the most
   destructive and deadly tornadoes.

   Many supercells never produce a tornado. They can produce damaging
   hail, extreme winds, frequent lightning, flash floods and can last for
   many hours.

   Duration: Tornadoes can last from several seconds to more than an hour.
   Most last less than 10 minutes.

   Damage: The new, Enhanced F-scale is becoming the standard for
   assessing tornado damage.

   Even with engineering guidelines, tornado wind speeds are based on
   calculations.

   Frequency: About 1,300 tornadoes strike the U.S. each year.

   The true average is not known due to recording mistakes, erroneous
   classification or under reporting.

   Path: Most tornadoes move from southwest to northeast, or west to east.
   Some change direction or even backtrack.

   Prediction: One or two days in advance, forecasters look for
   temperature and wind flow patterns which can cause enough moisture,
   instability, lift and wind shear for tornadic thunderstorms.
   Source: National Oceanic and Atmospheric Administration

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