This is America. Гришенкова Е.Г. - 108 стр.

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108
10. What is the goal of VORTEX2?
Understanding how tornadoes work
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.
Tornado season has begun in the United States. Violent storms struck several
parts of the country last week. The National Weather Service said there were ten
reports of tornadoes across the central and southern Plains states last Thursday.
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 three hundred twenty
kilometers an hour or more. In some cases, the resulting paths of damage can stretch
more than a kilometer wide and eighty kilometers long.
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 trees, cars or homes 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, eight
hundred tornadoes are reported nationwide each year.
The National Oceanic and Atmospheric Administration keeps records of
tornado sightings. It says tornadoes kill eighty people and injure one thousand five
hundred others nationwide in an average year.
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.

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