toefl写作范文下载之海水盐度

　　托福(TOEFL)考试写作范文材料:海水盐度

　　The Salinity of Ocean Waters 海水盐度

　　If the salinity of ocean waters is analyzed, it is found to vary onlyslightly from place to place. Nevertheless, some of these small changes areimportant. There are three basic processes that cause a change in oceanicsalinity. One of these is the subtraction of water from the ocean by means ofevaporation--- conversion of liquid water to water vapor. In this manner thesalinity is increased, since the salts stay behind. If this is carried to theextreme, of course, white crystals of salt would be left behind.

　　The opposite of evaporation is precipitation, such as rain, by which wateris added to the ocean. Here the ocean is being diluted so that the salinity isdecreased. This may occur in areas of high rainfall or in coastal regions whererivers flow into the ocean. Thus salinity may be increased by the subtraction ofwater by evaporation, or decreased by the addition of fresh water byprecipitation or runoff.

　　Normally, in tropical regions where the sun is very strong, the oceansalinity is somewhat higher than it is in other parts of the world where thereis not as much evaporation. Similarly, in coastal regions where rivers dilutethe sea, salinity is somewhat lower than in other oceanic areas.