编辑点评: GRE阅读的方法相信大家已经积累了很多了,本文就为大家提供一些GRE阅读的材料,大家来练一练,把平时学到的东西运用到实际中,也多多积累词汇和句子,提高自己的阅读能力。
每天做一些标准的GRE阅读练习,有助于大家在GRE考试的复习过程中不断地进行练习和总结。希望大家在进行GRE阅读练习时,充分运用平时所积累的知识,这样才能有效果。
The stratospheric ozone layer is not a completely uniform stratum, nor does it occur at the same altitude around the globe. It lies closest to the Earth over the poles and rises to maximum altitude over the equator. In the stratosphere, ozone is continuously being made and destroyed by natural processes. During the day the Sun breaks down some of the oxygen molecules to single oxygen atoms, and these reacting with the oxygen molecules that have not been dissociated, form ozone. However, the sunlight also breaks down ozone by converting some of it back to normal oxygen. In addition naturally occurring nitrogen oxides enter into the cycle and speed the breakdown reactions. The amount of ozone present at any one time is the balance between the processes that create it and those that destroy it.
Since the splitting of the oxygen molecules depends directly upon the intensity of solar radiation, the greatest rate of ozone production occurs over the tropics. However ozone is also destroyed most rapidly there, and wind circulation patterns carry the ozone-enriched upper layers of the atmosphere away from the equator. It turns out that the largest total ozone amounts are found at high latitudes. On a typical day the amount of ozone over Minnesota, for example, is 30 percent greater than the amount over Texas, 900 miles farther south. The density and altitude of the ozone layer also change with the seasons, the weather, and the amount of solar activity. Nevertheless, at any one place above the Earth s surface, the long-term averages maintained by natural processes are believed to be reasonably constant.
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