A satellite image of a swirling hurricane Men with gas masks and protective suits on The planetary rover Sojourner A large group of refugees huddled together







Tornadoes are rated on a scale of 0 to 5, with the strongest packing winds of 261 to 318 miles an hour (420 to 512 kilometers an hour), capable of lifting strong frame houses off foundations and ripping bark off trees.

The infamous 1925 Tri-State Tornado Outbreak—the longest-lasting on record—inspired a gradual, painstaking movement toward better tornado forecasting and more timely warnings. But gains have been measured in minutes, not hours. The average lead time for warnings, which for years was 6 minutes, is now up to almost 12.

Forecasters gather data from satellite imagery, radar, surface weather stations, weather balloons, and lightning detection networks. Doppler radar can detect strong rotation within a storm, information useful in predicting the likelihood of a tornado. NEXRAD (Next Generation Weather Radar) is a computerized Doppler system especially good at tracking and analyzing tornado-producing storms.

In the United States, two government agencies located in the heart of hurricane country provide research and timely guidance concerning hazardous weather. The National Severe Storms Laboratory conducts long-term research. The Storm Prediction Center, working with National Weather Service field offices around the country, offers short-term predictions and other guidance for those in the way of harm when the wind starts spinning.

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A photo of a tornado over Dallas, Texas   Click on this photo to enter photo gallery

A tornado survivor describes her experiences—including the sight of a flying cow.

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Two or more tornadoes may occur at the same time.

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