如果我們在(zai)兩(liang)(liang)根電(dian)極(ji)之間加很高的電(dian)壓，并把它(ta)們慢(man)慢(man)地靠近(jin)。當兩(liang)(liang)根電(dian)極(ji)靠近(jin)到(dao)一定的距離(li)時，在(zai)它(ta)們之間就會出現(xian)電(dian)火花，這就是所(suo)謂"弧光放(fang)電(dian)"現(xian)象。

雷雨云(yun)所(suo)產生的(de)(de)閃(shan)(shan)(shan)電(dian)(dian)(dian)(dian)，與(yu)上(shang)面(mian)(mian)所(suo)說的(de)(de)弧光(guang)放電(dian)(dian)(dian)(dian)非常(chang)相(xiang)似，只(zhi)不(bu)(bu)過(guo)閃(shan)(shan)(shan)電(dian)(dian)(dian)(dian)是(shi)轉瞬即逝，而電(dian)(dian)(dian)(dian)極之(zhi)間(jian)的(de)(de)火花卻(que)可(ke)以(yi)(yi)(yi)長(chang)時間(jian)存(cun)在。因(yin)為(wei)在兩根電(dian)(dian)(dian)(dian)極之(zhi)間(jian)的(de)(de)高(gao)電(dian)(dian)(dian)(dian)壓(ya)可(ke)以(yi)(yi)(yi)人(ren)為(wei)地(di)維持很(hen)久，而雷雨云(yun)中的(de)(de)電(dian)(dian)(dian)(dian)荷經放電(dian)(dian)(dian)(dian)后很(hen)難馬上(shang)補充(chong)。當聚集的(de)(de)電(dian)(dian)(dian)(dian)荷達到(dao)一定的(de)(de)數量(liang)時，在云(yun)內不(bu)(bu)同(tong)部(bu)位之(zhi)間(jian)或者(zhe)云(yun)與(yu)地(di)面(mian)(mian)之(zhi)間(jian)就形成了很(hen)強(qiang)的(de)(de)電(dian)(dian)(dian)(dian)場。電(dian)(dian)(dian)(dian)場強(qiang)度平均可(ke)以(yi)(yi)(yi)達到(dao)幾千伏(fu)特(te)/厘米，局部(bu)區(qu)域可(ke)以(yi)(yi)(yi)高(gao)達1萬(wan)伏(fu)特(te)/厘米。這(zhe)么強(qiang)的(de)(de)電(dian)(dian)(dian)(dian)場，足(zu)以(yi)(yi)(yi)把云(yun)內外的(de)(de)大氣層(ceng)擊(ji)穿，于(yu)是(shi)在云(yun)與(yu)地(di)面(mian)(mian)之(zhi)間(jian)或者(zhe)在云(yun)的(de)(de)不(bu)(bu)同(tong)部(bu)位之(zhi)間(jian)以(yi)(yi)(yi)及不(bu)(bu)同(tong)云(yun)塊之(zhi)間(jian)激發出耀眼的(de)(de)閃(shan)(shan)(shan)光(guang)。這(zhe)就是(shi)人(ren)們常(chang)說的(de)(de)閃(shan)(shan)(shan)電(dian)(dian)(dian)(dian)。

肉眼(yan)看(kan)到的(de)(de)一(yi)次(ci)(ci)(ci)閃(shan)(shan)電(dian)(dian)，其過(guo)程(cheng)(cheng)是(shi)很復雜的(de)(de)。當雷雨云(yun)(yun)移到某處時(shi)，云(yun)(yun)的(de)(de)中(zhong)(zhong)下(xia)部是(shi)強(qiang)大(da)負電(dian)(dian)荷(he)中(zhong)(zhong)心(xin)，云(yun)(yun)底相對的(de)(de)下(xia)墊(dian)面(mian)變成正電(dian)(dian)荷(he)中(zhong)(zhong)心(xin)，在(zai)云(yun)(yun)底與地(di)面(mian)間形(xing)成強(qiang)大(da)電(dian)(dian)場。在(zai)電(dian)(dian)荷(he)越(yue)積越(yue)多，電(dian)(dian)場越(yue)來越(yue)強(qiang)的(de)(de)情況下(xia)，云(yun)(yun)底首(shou)先(xian)(xian)出(chu)現大(da)氣被強(qiang)烈電(dian)(dian)離的(de)(de)一(yi)段氣柱，稱梯(ti)(ti)級(ji)先(xian)(xian)導。這(zhe)種電(dian)(dian)離氣柱逐級(ji)向地(di)面(mian)延伸(shen)，每級(ji)梯(ti)(ti)級(ji)先(xian)(xian)導是(shi)直(zhi)徑(jing)(jing)約(yue)5米(mi)、長50米(mi)、電(dian)(dian)流約(yue)100安培(pei)(pei)的(de)(de)暗(an)淡光(guang)(guang)(guang)柱，它(ta)以(yi)平均約(yue)150000米(mi)/秒的(de)(de)高(gao)速(su)度一(yi)級(ji)一(yi)級(ji)地(di)伸(shen)向地(di)面(mian)，在(zai)離地(di)面(mian)5―50米(mi)左(zuo)右時(shi)，地(di)面(mian)便突然向上(shang)回擊(ji)(ji)，回擊(ji)(ji)的(de)(de)通(tong)(tong)道(dao)是(shi)從地(di)面(mian)到云(yun)(yun)底，沿著上(shang)述(shu)梯(ti)(ti)級(ji)先(xian)(xian)導開辟出(chu)的(de)(de)電(dian)(dian)離通(tong)(tong)道(dao)。回擊(ji)(ji)以(yi)5萬(wan)公里/秒的(de)(de)更高(gao)速(su)度從地(di)面(mian)馳(chi)向云(yun)(yun)底，發出(chu)光(guang)(guang)(guang)亮無(wu)比(bi)的(de)(de)光(guang)(guang)(guang)柱，歷(li)時(shi)40微秒，通(tong)(tong)過(guo)電(dian)(dian)流超過(guo)1萬(wan)安培(pei)(pei)，這(zhe)即(ji)第(di)(di)一(yi)次(ci)(ci)(ci)閃(shan)(shan)擊(ji)(ji)。相隔百(bai)分之幾秒之后，從云(yun)(yun)中(zhong)(zhong)一(yi)根暗(an)淡光(guang)(guang)(guang)柱，攜帶巨大(da)電(dian)(dian)流，沿第(di)(di)一(yi)次(ci)(ci)(ci)閃(shan)(shan)擊(ji)(ji)的(de)(de)路徑(jing)(jing)飛(fei)馳(chi)向地(di)面(mian)，稱直(zhi)竄先(xian)(xian)導，當它(ta)離地(di)面(mian)5―50米(mi)左(zuo)右時(shi)，地(di)面(mian)再(zai)向上(shang)回擊(ji)(ji)，再(zai)形(xing)成光(guang)(guang)(guang)亮無(wu)比(bi)光(guang)(guang)(guang)柱，這(zhe)即(ji)第(di)(di)二次(ci)(ci)(ci)閃(shan)(shan)擊(ji)(ji)。接著又類(lei)似(si)第(di)(di)二次(ci)(ci)(ci)那(nei)樣產(chan)生(sheng)第(di)(di)三、四次(ci)(ci)(ci)閃(shan)(shan)擊(ji)(ji)。通(tong)(tong)常由3―4次(ci)(ci)(ci)閃(shan)(shan)擊(ji)(ji)構成一(yi)次(ci)(ci)(ci)閃(shan)(shan)電(dian)(dian)過(guo)程(cheng)(cheng)。一(yi)次(ci)(ci)(ci)閃(shan)(shan)電(dian)(dian)過(guo)程(cheng)(cheng)歷(li)時(shi)約(yue)0.25秒，在(zai)此短(duan)時(shi)間內，窄狹的(de)(de)閃(shan)(shan)電(dian)(dian)通(tong)(tong)道(dao)上(shang)要釋放巨大(da)的(de)(de)電(dian)(dian)能，因(yin)而形(xing)成強(qiang)烈的(de)(de)爆(bao)炸(zha)，產(chan)生(sheng)沖擊(ji)(ji)波(bo)，然后形(xing)成聲波(bo)向四周傳(chuan)開，這(zhe)就(jiu)是(shi)雷聲或說"打(da)雷"。