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Tham khảo tài liệu đề ôn thi đại học môn tiếng anh năm 2013-đề 6 , tài liệu phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | What exactly is a tornado? The general picture is familiar enough. The phenomenon usually occurs on a hot, sticky day with south wind and an ominous sky. From the base of a thunderhead, a funnel-shaped cloud extends a violently twisting spout toward the Earth. As it sucks in matter in its path, the twister may turn black, brown or even white. The moving cloud shows an almost continuous display of sheet lightning . It lurches along in a meandering path, usually northeast ward, at 25 to 40 miles per hour. Sometimes it picks up its finger from the earth for a short distance and then plants it down again. The funnel is very slender; its wake of violence averages about 400 yards wide. As the tornado approaches, it is heralded by a roar as a hundreds of jet planes or thousands of railroad cars. Its path is a path of total destruction. Buildings explode as they are sucked in by the tornado’s low-pressure vortex ( where the pressure drop is as much as 10 percent ) and by its powerful whirling winds ( estimated at up to 500 miles per hour ). The tornado’s lifetime is as brief as it is violent. Within a few tens of miles ( average about 16 miles), it spends its force and suddenly disappears.
