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科学揭秘暴风雨前的宁静Haveyoueverspentanafternooninthebackyard,maybegrillingorenjoyingabasketballgame,whensuddenlyyounoticethateverythinggoesquiet?Theairseemsstillandcalm—eventhebirdsstopsingingandquicklyreturntotheirnests.你可曾有过这样的经历:某天下午正在后院烧烤或者打篮球的时候,突然发现一切瞬间变得安静下来,空气似乎凝结了,连鸟儿也不再歌唱而转身躲回窝里去了?Afterafewminutes,youfeelachangeintheair,andsuddenlyalineofcloudsominouslyappearsonthehorizon—cloudswithalookthattellsyoutheyaren'tfoolingaround.Youquicklydashinthehouseandnarrowlymissthefirstfatraindropsthatfallrightbeforethedownpour.Atthismoment,youmightstopandaskyourself,Whywasitsocalmandpeacefulrightbeforethestormhit?几分钟后,你感觉到空气中有所变化,忽然之间一排乌云出现在地平线上,让人有种不祥的预感——乌云的样子好像是在告诉你它们可不只是做做样子而已。你急冲进屋里,险些被瓢泼大雨前掉下的大雨点砸个正着。此时,你也许会停下来问自己:“为什么暴风雨来临前是那么宁静安详呢?”It'sanintriguingphenomenonthatpeoplehaverecognizedforcenturies,butwhatonearthcausesthiscalm?Andthereisanoldphrasecalmbeforethestorm,oftenusedinitsfigurativemeaning—aquietperiodjustbeforeagreatactivityorexcitement.Accordingtoourownexperience,weknowthereisliterallycalmbeforethestorm.Butwhatcausesthiscalm?Andisitalwayscalmbeforethestorm?Let'shearwhatscientistshavetosay.人们在几个世纪前就已经发现这个神秘的现象了,但究竟是什么原因造成暴风雨前的这番宁静呢?古谚有云“暴风雨前的宁静”,这通常用在比喻的修辞中,描述热闹的活动以及情绪亢奋前的一段安静时间。根据自身的经验,我们也知道事实上暴风雨来临前也确实是宁静的。但这是什么原因造成的呢?暴风雨前总是平静么?让我们来听听科学家们的解释。Theanswerissometimesthereis;sometimesthereisn't.Undertherightconditions,aneerieorpeacefulcalmwillbefallyourpicnicbeforeastormmovesin.Otherstormsskipthecalmandproudlyannouncetheirpresencebyinstantlyunleashingbadweather.答案是:有时如此,有时则不然。在特定的情况下,暴风雨来临前会出现令人恐惧的或是祥和的寂静,降临在你野餐的时候。其他种类的暴风雨则直接跳过寂静的步骤,犹如在骄傲地宣布自己的存在一般,瞬间风雨大作。Aperiodofcalmhappensinaparticularkindofstorm,thesimplestkindofstorm—asingle-cellthunderstorm.Inthistypeofthunderstorm,thereisusuallyonlyonemainupdraft,whichiswarm,humidairanddrawnfromplacesneartheground.Stormsneedwarmandmoistairasfuel,sotheytypicallydrawthatairinfromsurroundingenvironment.Stormscandrawintheairthatfittheirneedfromalldirections—evenfromthedirectioninwhichthestormistraveling.平静时期出现在一种特定的暴风雨中,即最简单的暴风雨——单细胞雷暴。在这种类型的雷暴中,通常只有一种主要的上升气流,即暖湿气流从地面升起。温暖湿润的空气是暴风雨形成的必要条件,所以通常会把它们从周围的环境中吸进来。雷暴能够从周围任何一个方向吸进自己所需的暖湿空气——甚至是处在雷暴运动方向上的空气也被吸了进来。Asthewarm,moistairispulledintoastormsystem,itleavesalow-pressurevacuuminitswake.Therisingairmeetsthecolddryairthathasalreadyexistedinthestormclouds,thusthetemperatureofthewarmairdrops,andthewatervaporinitcondensesintotinydropletsthatareapreconditionofrain.Thesedropletsclumpupandbuildonlargerparticleslikedust,untiltheygrowlargeenoughtoformraindrops.由于暖湿空气被吸进雷暴中,雷暴经过之处就形成了低气压真空带。正在上升的暖湿气流遇到雷暴云层中原本就存在的干燥冷空气,此时热空气的温度下降,里面的水分凝结成小的水滴——这是形成雨水的前提。这些水滴不断聚集,包裹住一些大的颗粒(如灰尘),当水滴凝聚到足够大时就会形成降雨。Thiswarm,moistairkeepsmovingupwards,butitbecomescolderanddrierduringitstripthroughcloudwhichinvolvescoolingandcondensation.Whenitreachesthetopofthecloudmass,theairgetsspitoutatthetop.Thisairissentrollingoutoverthebig,anvil—shapedheadofthethunderclouds.Fromthere,theairdescends—drawnbacktowardloweraltitudesbytheveryvacuumitsdeparturecreatedinthefirstplace.Warmanddryairisrelativelystable,andonceitblanketsaregion,itstabilizesthatairinturn.Thiscausesthecalmbeforeastorm.暖湿气流持续上升,但当穿越云层时经历气温下降水汽凝结的过程,暖湿气流开始冷却并变得干燥。当暖湿气流达到云层顶部时,会被挤出云层,这股气流会翻过巨大的铁砧形雷雨云。此时,气流开始下沉——被其自身上升时所形成的真空带吸回低处。干热的气流相对稳定,一旦它覆盖了某一个地区,就会使该区域的气流稳定下来,这就造成了暴风雨前的平静。Mostthunderstorms,though,don'tstartwithacalm.That'sbecausemostareactuallyclustersofstormswithcomplexwindpatterns.There'ssomuchairmovingupanddowninthevicinityofthesestormclustersthatthecalmbeforethestormneverhappens.Andinstead,beforethestorm,itmightbereallywindy!尽管如此,大多数的雷暴来临前并不是一片宁静。这是因为它们大多是风向复杂的暴风雨团。在暴风雨团的周围,有很多气流频繁上下流动,这就使得这种类型的暴风雨来临前毫无平静可言。相反,这种类型的暴风雨来临前往往会狂风大作。
本文标题:科学揭秘-暴风雨前的宁静-
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