This is SCIENCE IN THE NEWS in VOA Special English. I'm Bob Doughty.
And I'm Pat Bodnar. This week -- the science of tornadoes.
For many parts of the United States, the spring season brings with it tornadoes.
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 severe wind storms are also known as twisters.
The most severe tornadoes can reach wind speeds of three hundred twenty kilometers an hour or more. In some cases, damage paths can stretch more than one kilometer wide and eighty kilometers long.
With a tornado, bigger does not necessarily mean stronger. Large tornadoes can be very 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. And they can also destroy anything in their path.
Tornadoes have been observed on every continent except Antarctica.
But weather scientists say the country where they are most common is the United States. The United States has more than one thousand tornadoes a year.
Last year, twelve of them resulted in deaths. In all, thirty-eight people were killed.
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.
Weather scientists say Texas is the state with the greatest number of tornadoes. But the state with the most tornadoes in relation to area is Florida. In Florida, tornadoes often develop along the edges of severe ocean storms.
Tornadoes can happen any time of the year. But they happen most often in March, April and May. There is a second high season in November.
In the spring, warm air moves north and mixes with cold air remaining from winter. In November, 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 a small room, without windows, in the middle of the lowest part of a building.
People driving during a tornado are told to find low ground and lie 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.
The deadliest United States tornado on record is the Tri-State Tornado of March eighteenth, nineteen twenty-five. It tore across Missouri, Illinois and Indiana. About seven hundred 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 third and fourth, nineteen seventy-four, set a national record. It is remembered as the "Super Outbreak."
One hundred forty-eight tornadoes struck during a twenty-four-hour period. More than three hundred people were killed and six thousand others were injured.
One tornado that was especially destructive hit Xenia, Ohio. The sound you are about to hear comes from the Web site ohiohistory.org.
No two tornadoes look exactly alike. 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 scientists 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.
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.
The tornado reporting system involves watches and warnings. When people are told that a tornado watch is in effect, that 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 nighttime 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 thirty-two to sixty-four kilometers per hour. But they have been reported to move in other directions and as fast as one hundred seventeen kilometers an hour.
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 University of Chicago weather expert who developed this system in the nineteen seventies.
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.
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.
Some chasers are part of weather research teams. Others do it to help document storms and warn the public. Still others do it just because it is their idea of fun.
Storm chasers usually drive large vehicles to areas of severe weather. They follow storms for long distances. For some, the appeal of a tornado is to get closer and take better pictures than others have, without getting killed in the process.
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 beaches 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.
The hurricane season officially begins on June first and ends on November thirtieth.
SCIENCE IN THE NEWS was written by Brianna Blake. Cynthia Kirk was our producer. I'm Bob Doughty.
And I'm Pat Bodnar. Read and listen to our programs at WWW.51VOA.COM. And if you have a science question, send it to firstname.lastname@example.org. We might be able to answer it on our show. Join us again next week for more news about science in Special English on the Voice of America.