News / USA

    Tornado Factbox

    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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