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  3. One of nature’s most destructive forces is the tornado, a violent windstorm that takes the shape of a rotating column of air. To

One of nature’s most destructive forces is the tornado, a violent windstorm that takes the shape of a rotating column of air. To

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问题    One of nature’s most destructive forces is the tornado, a violent windstorm that takes the shape of a rotating column of air. Tornadoes almost always occur in conjunction with severe thunderstorms that produce high winds, heavy rainfall and damaging hail. Though their cause is unknown, tornadoes are believed to be the result of the convergence of strong upward wind currents inside a storm with upper level winds above the storm; the greater the air contrast, the more violent the storm will be. The United States has more tornadoes, approximately 750 tornadoes reported each year, than any other country in the world because this is where arctic and tropical air masses most frequently converge.
   This unpredictability makes accurate tornado forecasts difficult. Though it is possible to determine when a tornado is apt to occur, actual tornado warnings are issued only when a tornado has been sighted or reported on radar. Radar can be used to guess the storm’s’ likely path, its speed and the intensity of the storm. But conventional radar has limitations. An advanced form of radar, known as Doppler, has the ability to detect the first steps in the formation of a tornado. Unlike conventional radar, Doppler tracks a thunderstorm’s rotating wind system which usually precedes the development of a tornado. As a result, Doppler has provided forecasters with the ability to issue tornado warnings as much as 20 minutes prior to a storm’s touchdown, compared to a warning of less than 2 minutes by visual sighting. Though tornadoes remain one of nature’s most violent forces, the use of radar and advanced warning systems has substantially narrowed their paths of destruction.
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答案 自然界最具破坏性的力量之一是龙卷风，它是一种旋转的柱状强烈风暴。龙卷风，几乎总是与强烈的雷暴并列发生，雷暴能引发大风、暴雨和破坏性的冰雹。虽然它们的起因尚未知晓；但人们认为龙卷风是由暴风雨中上升的强气流与暴风上方的风的汇聚造成的；气流矿比越明显，龙卷风的威力也就越大。美国比世界上其他任何地方的龙卷风都要多，每年大约有750起龙卷风事件报道，这是因为北极气流和热带气流极频繁地在美国汇聚。 这种不可预知性使得人们很难精确预报龙卷风。虽然人们可能预报是否会有龙卷风发生，但真正的龙卷风警报只有在被雷达探测到以后才能发出。雷达可以被用来探测龙卷风的大致路径、速度和风暴的强度。但传统雷达有一些局限性。多普勒——雷达的一种高新形式——有探测龙卷风初步形成的能力。与传统雷达不同的是，多普勒能追寻到一个雷暴旋风系统，这个系统通常发生在龙卷风之前。因此，多普勒能够提供一些预报，而且能够在龙卷风着陆20分钟之前发出警报，它在得到可视图像后两分钟内就可以发出警报。虽然龙卷风仍然是自然界最强烈的力量之一，雷达和先进预警机制的使用已经大大减小了它的破坏性。
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