四川学位英语真题

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Most people know that water is unevenly distributed over the earth's surface in oceans, rivers, and lakes. Few realize, however, how very uneven the distribution actually is. It is necessary to think of the total amount of water on Earth, the areas where the water occurs, and the long-term importance of the findings. The oceans of the world cover 140 million square miles of the Earth's surface and the average depth of the ocean basin is about 12,500 feet. If the basins were shallow, seas would spread far onto the continents. Dry land areas would consist mainly of a few island groups with high mountain ranges rising above the sea. Considered as a continuous body of fluid, the atmosphere is another kind of ocean. Yet, in view of the total amount of rain and snow on land areas in the course of a year, one of the most amazing water facts is the very small amount of water in the atmosphere at any given time. The volume of the lower seven miles of the atmosphere is roughly four times the volume of the world's oceans. But the atmosphere contains very little water. It is chiefly in the form of invisible vapor, some of which is carried over land by air currents. If all vapor suddenly fell from the air onto the Earth's surface, it would form a layer only about one inch thick. A heavy rainstorm on a given area may use up only a small percentage of the water from the air mass that passes over. How, then, can some land areas receive more than 400 inches of rain per year? How can several inches of rain fall during a single storm in a few minutes or hours? The answer is that rain-yielding air masses are in motion, and as the drying air mass moves on, new moist air takes its place.【缺少答案,请补充】