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  3. A weather map is an important tool for geographers. A succession of three or four maps presents a continuous picture of weather

A weather map is an important tool for geographers. A succession of three or four maps presents a continuous picture of weather

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问题    A weather map is an important tool for geographers. A succession of three or four maps presents a continuous picture of weather changes. Weather forecasters are able to determine the speed of air masses and fronts; to determine whether an individual pressure area is deepening or becoming shallow and whether a front is increasing or decreasing in intensity~ They are also able to determine whether an air mass is retaining its original characteristics or taking on those of the surface over which it is moving. Thus, a most significant function of the map is to reveal a synoptic picture of conditions in the atmosphere at a given time.
   All students of geography should be able to interpret a weather map accurately. Weather maps contain an enormous amount of information about weather conditions existing at the time of observation over a large geographical area. They reveal in a few minutes what otherwise would take hours to describe. The United States Weather Bureau issues information about approaching storms, floods, fronts and all climatic conditions in general. Twice a month it issues a 30-day "outlook" which is a rough guide to weather conditions likely to occur over broad areas of the United States. These 30-day outlooks are based upon an analysis of the upper air levels which often set the stage for the development of air masses, fronts, and storms.
   Considerable effort is being exerted today to achieve more accurate weather predictions. With the use of electronic instruments and earth satellites, enormous gains have taken place recently in identifying and tracking storms over regions which have but few meteorological stations. Extensive experiments are also in progress for weather modification studies. But the limitations of weather modification have prevented meteorological results except in the seeding of super-cooled, upslope mountainous winds which have produced additional orographic precipitation on the windward side of mountain ranges. Nevertheless they have provided a clearer understanding of the fundamentals of weather elements.
The 30-day forecast is determined by examining ______.
选项 A、daily weather maps
B、upper air levels
C、satellite reports
D、changing fronts
答案B
解析 30-day forecast和30-day outlooks；analysis与examining是两对同义词，因此定位到第二段最后一句These 30-day outlooks are based upon an analysis of the upper air levels which often set the stage for the development of air masses，fronts，and storms．(这些以三十天为周期的气象图是在对上层空气水平的分析基础上编制的)，所以选[B] 。
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