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发布时间：2023-10-10 10:46:59 来源：文档文库

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A(2019年江西省名校高三联合考试Haveyoueverspentanafternooninthebackyard,maybegrillingorenjoyingabasketballgame,whensuddenlyyounoticethateverythinggoesquiet?Thereisanoldphrase“calmbeforethestorm”，oftenusedinasituation—aquietperiodjustbeforeagreatactivityorexcitement.Accordingtoourownexperience,weknowthereisactuallycalmbeforethestorm.Butwhatcausesthiscalm?Andisitalwayscalmbeforethestorm?Aperiodofcalmhappensinaparticularkindofstorm,thesimplestkindofstorm—asinglecellthunderstorm.Inthistypeofthunderstorm,thereisusuallyonlyonemainstreamofcurrent,whichiswarm,dampairanddrawnfromplacesneartheground.Stormsneedwarmanddampairasfuel,sotheytypicallydrawthatairinfromsurrounding
environment.Stormscandrawintheairthatfittheirneedfromalldirections—evenfromthedirectioninwhichthestormistraveling.Asthewarm,dampairispulledintoastormsystem,itleavesalowpressurevacuum(真空comingafter.Therisingairmeetsthecolddryairthathasalreadyexistedinthestormclouds,thusthetemperatureofthewarm,dampairdrops,andthewatervaporinitchangesintotinydropsthatareapreconditionofrain.Thesedropsaccumulateandbuildonlargerparticles(颗粒likedust,untiltheygrowlargeenoughtoformraindrops.Thiswarm,dampairkeepsmovingupwards,butitbecomescooleranddrierduringitstripthroughcloud.Whenitreachesthetopofthecloud,theairissqueezedoutatthetop.Thisairissentrollingoutoverthebigthunderclouds.Fromthere,theairgoesdown.Warmanddryairisrelativelystable,andonceitcoversaregion,thatair,inturn,causesthecalmbeforeastorm.Mostthunderstorms,though,don'tstartwithcalm.That'sbecausemostareactuallygroupsofstormswithcomplexwindpatterns.There'ssomuchairmovingupanddownstormgroupsthatthecalmbeforethestormneverhappens.Instead,beforethestorm,itmightbereallywindy!1．Whichbestfitsthedescriptionofaparticularkindofstorm?A．Athunderstormwithasingleshape.B．Athunderstormwithoutstrongwinds.

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