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 cloud 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 windstorms are also known as twisters.

　　Tornadoes have been observed on every continent except Antarctica.But weather experts say they are most common in the United States.Each year，the United States has more than one thousand tornadoes.

　　These storms can happen at any time of the year.But most happen from late winter to the middle of summer.There is a second high season in November.

　　During 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.Weather experts operate warning systems to tell people about possible tornadoes.But the storms often move too fast for people to flee.Last year，tornadoes killed more than one hundred people in the United States.

　　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.

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