六、阅读与回答问题
What is a tornado (龙卷风)? A tornado is a spinning (旋转的) tower of air that goes from the sky to the ground. Tornadoes can cause huge amounts of damage and once they start, they cannot be stopped.
What causes tornadoes?
Tornadoes happen in many parts of the world. The United States experiences around 1,000 tornadoes and has some of the most powerful ones a year.
The exact cause of tornadoes is not fully known. They form during thunderstorms. Inside the storm clouds, warm and humid (湿热的) air rises and cool air falls. The warm and cool air begin to spin horizontally (水平地). Eventually, this spinning can become vertical and push itself out of the storm clouds as a funnel (漏斗). The moment the funnel of spinning air touches the ground, it becomes a tornado.
Levels of tornadoes
There are three levels of tornadoes: weak, strong and violent.
Around two thirds of tornadoes are weak. These tornadoes have wind speeds that are below 177 km/h and they usually last no longer than ten minutes.
Strong tornadoes make up about one third of all tornadoes. They have wind speeds that are between 178 km/h and 322 km/h and can last about twenty minutes.
The most dangerous tornadoes are violent tornadoes. They can last for longer than an hour and have wind speeds that are higher than 322 km/h.
51. What is a tornado?
A spinning tower of air that goes from the sky to the ground.
52. Which country do tornadoes often hit according to the passage?
The United States.
53. How many levels of tornadoes are there?
Three.
54. What are the wind speeds of strong tornadoes?
Between 178 km/h and 322 km/h.
55. What can you do when there is a violent tornado? (请自拟一句话作答)
We can keep calm and stay at home.
