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2015考研英语作文素材精选
39 TheSalinityofOceanWaters
Ifthesalinityofoceanwatersisanalyzed, itisfoundtovaryonlyslightlyfromplacetoplace. Nevertheless, someofthesesmallchangesareimportant. Therearethreebasicprocessesthatcauseachangeinoceanicsalinity. Oneoftheseisthesubtractionofwaterfromtheoceanbymeansofevaporation--- conversionofliquidwatertowatervapor. Inthismannerthesalinityisincreased, sincethesaltsstaybehind. Ifthisiscarriedtotheextreme, ofcourse, whitecrystalsofsaltwouldbeleftbehind.
Theoppositeofevaporationisprecipitation, suchasrain, bywhichwaterisaddedtotheocean. Heretheoceanisbeingdilutedsothatthesalinityisdecreased. Thismayoccurinareasofhighrainfallorincoastalregionswhereriversflowintotheocean. Thussalinitymaybeincreasedbythesubtractionofwaterbyevaporation, ordecreasedbytheadditionoffreshwaterbyprecipitationorrunoff.
Normally, intropicalregionswherethesunisverystrong, theoceansalinityissomewhathigherthanitisinotherpartsoftheworldwherethereisnotasmuchevaporation. Similarly, incoastalregionswhereriversdilutethesea, salinityissomewhatlowerthaninotheroceanicareas.
Athirdprocessbywhichsalinitymaybealteredisassociatedwiththeformationandmeltingofseaice. Whenseawaterisfrozen, thedissolvedmaterialsareleftbehind. Inthismanner, seawaterdirectlymaterialsareleftbehind. Inthismanner, seawaterdirectlybeneathfreshlyformedseaicehasahighersalinitythanitdidbeforetheiceappeared. Ofcourse, whenthisicemelts, itwilltendtodecreasethesalinityofthesurroundingwater.
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