Last updated on: May 22, 2011 8:00 PM
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.