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2. What Is a Tornado? What exactly is a tornado? The general picture is familiar enough. The phenomenon usually occurs on a hot. sticky day with south winds 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 occasionally (over snow) even white. The moving cloud shows an almost continuous display of sheet lightning. It lurches along in a meandering path, usually northeastward, 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 of hundreds of jet planes or thousands of railroad cars. Its path is a path of total destruction. Buildings literally 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. 3. Earthquakes Earthquake is a sudden, violent shaking of the earth's surface. It's a kind of crustal movement form of expression. Earthquake is regarded as one of the most damaging forces known to man: since records began to be written down, it has been estimated that earthquake-related fatalities have been numbered in the millions, and that earthquake-related destruction has been beyond count. The great majority of all earthquakes occur in two specific geographic areas. One such area surrounds the Pacific Ocean and its contiguous land masses. The other extends from the East India to the Atlas Mountains, including the Himalayas, Iran, Turkey, and the Alpine regions. It is in these two great belts or zones that ninety percent of all earthquakes take place. The text refers to two of the strongest quakes in San Francisco. As to our country, a large number of earthquakes happened in North China and Northwest China. The powerful earthquake of Tang Shan happened in the city of Tang Shan, Hebei Province on July 28th, 1976, which caused more than 240 000 deaths and the whole city was destroyed. (责任编辑：admin) |