08全国职称英语：卫生类英语概括大意(17)

08全国职称英语：卫生类英语概括大意(17)

?SVoltsfromtheSky

?SLightninghascausedaweandwondersinceoldtimes.AlthoughBenjaminFranklindemonstratedlightningasenormouselectricaldischargemorethan200yearsago,manypuzzlesstillsurroundthispowerfulphenomenon.

Lightningisgeneratedwhenelectricalchargesseparateinrainclouds,thoughprocessesarestillnotfullyunderstood.Typically,positivechargesbuildatthecloudtop,whilethebottombecomesnegativelycharged.Inmostinstancesofcloud-to-groundlightning,thenegativelychargedlowerportionofthecloudrepelsnegativelychargedparticlesontheground’ssurfaces,makingitbecomepositivelycharged.Thepositivechargeonthegroundgathersatelevatedpoints.

Aflowofelectronsbeginsbetweenthecloudandearth.Whenthevoltagechargebecomeslargeenough,itbreaksthroughtheinsulatingbarrierofair,andelectronszigzagearthward.Weseethedischargeaslightning.

Lightningcanoccurwithinacloud,betweenclouds,orbetweencloudsandtheground.Thefirstvariety,intra-cloudlightning,isthemostfrequentbutisoftenhiddenfromourview.Cloud-to-groundlightning,makingupabout20percentoflightningdischarges,iswhatweusuallysee.Lightningcomesinseveralforms,includingsheet,ribbon,andball.Intra-cloudlightningcanilluminateacloudsoitlookslikeawhitesheet,henceitsname.Whencloud-to-groundlightningoccursduringstrongwinds,theycanshiftthelightningchannelsideways,soitlookslikearibbon.Theaveragelightningstrikeismorethan3mileslongandcantravelatatenthofthespeedoflight.Balllightning,therarestandmostmysteriousform,derivesitsnamefromthesmallluminousballthatappearsneartheimpactpoint,moveshorizontally,andlastsforseveralseconds.

Thunderisgeneratedbythetremendousheatreleasedinalightningdischarge.Temperaturesnearthedischargecanreachashighas50,000°Fwithinthousandthsofasecond.Thissuddenheatingactsasanexplosion,generatingshockwaveswehearasthunder.

About2,000thunderstormsareoccurringintheworldatanytime,generatingabout100lightingstrikeseverysecond,or8milliondaily.WithintheUnitedStates,lightningstrikesareestimatedat20millionayear,orabout22,000perday.Youhavea1-in-600,000chanceofbringstruckbylightningduringyourlifetime.

Lightningcanstriketwiceormoreinthesamespot.TheEmpireStateBuildinginNewYorkisstruckbylightningabouttwodozentimesannually.

Youcanmeasurehowfaryouarefromalightningstrikebycountingthesecondsbetweenviewingtheflashandhearingthebang,andthendividingbyfive.Thisapproximatesthemileage.

练习:1-4题的要求是,从第一个方框的六个选项A、B、C、D、E、F中选出四个选项为注明的段落各配一个合适的小标题。5-8题的要求是,从第二个方框的六个选项A、B、C、D、E、F中选出四个选项完成每个句子。

1.Paragraphs2and3 2.Paragraph4 3.Paragraph5 4.Paragraph6

ACloud-to-groundlightningoccurringintheU.S. BTypesoflightning CCauseoflightning

DDifferencesbetweenthunderandthunderstorm

EFrequenciesofthunderstormsoccurringintheworldandtheU.S. FShockwavesasthunder

5.Inmostcasesofcloud-to-groundlightning,theground’ssurface… 6.Oneformoflightningthat…isballlightning…

7.Cloudlightninglookslikearibbonwhenitslightingchannel… 8.Althoughnotfullyunderstandingprocessesoflightning,man… Aoccursmostinfrequently. Bisshiftedsidewaysbystrongwinds. Cisoftenhiddenfromourview.

Disequippedwithagoodknowledgeofvariousformsoflightning. Eisestimatedat20millionsayear. Fispositivelycharged. KEYS:CBFEFABD

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