BARBARA KLEIN: This is SCIENCE IN THE NEWS in VOA Special English. I'm Barbara Klein.

BOB DOUGHTY: And I'm Bob Doughty. This week, we will tell about the science of tornadoes. Tornadoes have been observed in many parts of the world. But the storms are most often found in the United States.

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BARBARA KLEIN: Tornado season has begun in the United States. More than 60 tornadoes were reported in the country's southeast during the last weekend in April. One tornado measuring more than a kilometer wide struck the state of Mississippi. It destroyed homes, blocked roads and cut off electricity.

The storms were blamed for ten deaths in Mississippi, and two others in nearby Alabama. Weather experts believe it was the most intense tornado event since 2008.

BOB DOUGHTY: A tornado is a violently turning tube of air suspended from a thick cloud. It extends from a thunderstorm in the sky down to the ground. The shape is like a funnel: wide at the top, narrower at the bottom.

Tornadoes form when winds blowing in different directions meet in the clouds and begin to turn in circles. Warm air rising from below causes the wind tube to reach toward the ground. Because of their circular movement, these windstorms are also known as twisters.

The most severe tornadoes can reach wind speeds of 320 kilometers an hour or more. In some cases, the resulting paths of damage can stretch more than a kilometer wide and 80 kilometers long.

BARBARA KLEIN: With a tornado, bigger does not necessarily mean stronger. Large tornadoes can be weak. And some of the smallest tornadoes can be the most damaging. But no matter what the size, tornado winds are the strongest on Earth. Tornadoes have been known to carry homes, cars and trees from one place to another. They can also destroy anything in their path.

Tornadoes have been observed on every continent except Antarctica. But experts say they are most commonly seen in the United States. On average, 800 tornadoes are reported nationwide each year.

The National Oceanic and Atmospheric Administration keeps records of tornado sightings. It says tornadoes kill 80 people and injure 1,500 others nationwide in an average year.

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BOB DOUGHTY: Tornadoes are observed most often in the central part of the United States, where the land is mostly flat. The area where the most violent tornadoes usually happen is known as "Tornado Alley." This area is considered to extend from north-central Texas to North Dakota.

Tornadoes can happen any time of the year. But most happen from late winter to the middle of summer. In some areas, there is a second high season in autumn.

BARBARA KLEIN: Tornado seasons are the result of wind and weather patterns. During spring, warm air moves north and mixes with cold air remaining from winter. In autumn, the opposite happens. Cold weather moves south and combines with the last of the warm air from summer.

Tornadoes can strike with little or no warning. Most injuries happen when flying objects hit people. Experts say the best place to be is in an underground shelter, or a small, windowless room in the lowest part of a building.

People driving during a tornado are told to find low ground and lay flat, facedown, with their hands covering their head. People in the path of a tornado often have just minutes to make life-or-death decisions.

BOB DOUGHTY: The deadliest American tornado on record is the Tri-State Tornado of March 18th, 1925. It tore across Missouri, Illinois and Indiana. About 700 people were killed.

A "tornado outbreak" is often defined as six or more tornadoes produced by the same weather system within a day. But the outbreak of April 3rd and 4th, 1974, set a national record. It is remembered as the "Super Outbreak." One hundred forty-eight tornadoes struck during a 24-hour period. More than 300 people were killed and nearly 6,000 others injured.

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.

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BARBARA KLEIN: No two tornadoes look exactly the same. And no two tornadoes act the same way.

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.

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.

BOB DOUGHTY: 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.

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.

Tornadoes that form over water are called waterspouts. But tornadoes cover a much smaller area than hurricanes, which form over oceans.

Tornadoes can be measured using wind speed information from Doppler radar systems. Tornadoes usually travel in a northeasterly direction with a speed of 32 to 64 kilometers an hour. But they have been reported to move in other directions and as fast as 117 kilometers an hour.

BARBARA KLEIN: 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 1970s.

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 100 meters are called an F-five.

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BOB DOUGHTY: 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 increasingly popular on television and on the Internet.

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.

This month, more than 100 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.

Last year, the project collected information about a tornado in southeastern Wyoming. Researchers also observed several other powerful storms that were not tornadoes.

BARBARA KLEIN: 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.

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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BOB DOUGHTY: This SCIENCE IN THE NEWS was written by Brianna Blake, who was also our producer. I'm Bob Doughty.

BARBARA KLEIN: And I'm Barbara Klein. Read and listen to our programs at voaspecialenglish.com. And if you have a science question, send it to special@voanews.com. We might answer it on our program. Join us again next week for more news about science, in Special English, on the Voice of America.

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（来源：VOA 编辑：陈丹妮）