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1. One tornado that was especially destructive hit Xenia, Ohio. The sound you are about to hear comes from the Web site www.xeniatornado.com. It is one man's recording of the tornado moving closer. No two tornadoes look exactly the same. And no two tornadoes act the same way.
2. It takes the right combination of wind, temperature, pressure and humidity to create even a weak tornado. Weather experts can identify these conditions. And, when they observe them, they can advise people that tornadoes might develop. But they are not able to tell exactly where or when a tornado will hit. Tornado warnings still depend in large part on human observations.
3. Usually a community will receive a warning at least a few minutes before a tornado strikes. But each year there are some surprises where tornadoes develop when they are least expected.
4. The tornado reporting system involves watches and warnings. A tornado watch means tornadoes are possible in the area. A tornado warning means that a tornado has been seen. People are told to take shelter immediately.
5. Yet tornadoes can be difficult to see. Sometimes only the objects they are carrying through the air can be seen. Some night-time tornadoes have been observed because of lightning strikes nearby. But tornadoes at night are usually impossible to see.
6. Tornadoes that form over water are called “waterspouts”. But tornadoes cover a much smaller area than hurricanes, which form over oceans.
7. Tornadoes can be measured using wind speed information from Doppler radar systems. Tornadoes usually travel in a northeasterly direction with a speed of thirty-two to sixty-four kilometers an hour. But they have been reported to move in other directions and as fast as one hundred seventeen kilometers an hour.
8. In the United States, the force of a tornado is judged by the damage to structures. Scientists inspect the damage before they estimate the severity of a tornado. They measure tornadoes on the Fujita scale. Ted Fujita was a weather expert who developed this system in the nineteen seventies.
9. There are six levels on the Fujita scale. Tornadoes that cause only light damage are called an F-zero. Those with the highest winds that destroy well-built homes and throw vehicles more than one hundred meters are called an F-five.
10. Some people make a sport out of watching and following tornadoes. They are called tornado chasers or storm chasers. Their work can be seen in the extreme weather videos that are increasinglypopular on television and on the Internet.
11. Some chasers do it just because it is their idea of fun. Others do it to help document storms and warn the public. Still others are part of weather research teams.
12. This month, more than one hundred researchers began deploying radar and other scientific instruments across America’s Great Plains. They are hoping to surround tornadoes in the second and final year of an international project known as VORTEX2. The goal of the project is to examine in detail how tornadoes are formed and the kinds of damage they cause.
13. Last year, the project collected information about a tornado in southeastern Wyoming. Researchers also observed several other powerful storms that were not tornadoes.
14. The National Weather Service says the United States gets more severe weather than any other country. For one thing, it is also bigger than most other countries. And it has many different conditions that create many different kinds of weather.
15. There are seacoasts and deserts, flatlands and mountains. The West Coast is along the Pacific Ocean, which is relatively calm. The East Coast is along the Atlantic Ocean, which is known for its hurricanes. These strike mainly the southeastern states.
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