環天頂弧(英語:circumzenithal arc或(huo)circumzenith arc,縮(suo)寫:CZA)或(huo)稱(cheng)布拉維弧(Bravais'arc),是一種(zhong)在(zai)外觀上類似彩(cai)(cai)虹的光(guang)學(xue)現象;但它(ta)的形成是太陽光(guang)從水平方向通過(guo)冰晶(一般位于卷云內)后折射而形成,而非雨滴。一般稱(cheng)之為“倒掛彩(cai)(cai)虹”或(huo)“天空的微笑(xiao)”。
環天(tian)頂弧(hu)形(xing)成(cheng)于和(he)太陽(yang)同一側的(de)(de)(de)天(tian)空,并(bing)且(qie)以(yi)不超(chao)過四(si)分之一圓周的(de)(de)(de)長度環繞(rao)天(tian)頂。它的(de)(de)(de)最(zui)內側顏(yan)(yan)色是(shi)藍色,最(zui)外側則是(shi)紅色,排列順序與彩(cai)虹相反。環天(tian)頂弧(hu)是(shi)其中一個最(zui)明(ming)亮和(he)最(zui)多種(zhong)顏(yan)(yan)色的(de)(de)(de)暈。它的(de)(de)(de)顏(yan)(yan)色比彩(cai)虹單純,這是(shi)因(yin)為在形(xing)成(cheng)過程中重(zhong)疊(die)的(de)(de)(de)顏(yan)(yan)色較(jiao)少。而它給人的(de)(de)(de)第一印象就是(shi)上下顛倒的(de)(de)(de)彩(cai)虹。
環天(tian)(tian)頂(ding)弧(hu)曾經被(bei)(bei)稱為“天(tian)(tian)空(kong)的(de)(de)微笑(xiao)”。因為它的(de)(de)形(xing)成位置相(xiang)當高,很少被(bei)(bei)人注意到,但實(shi)際上它是相(xiang)對較常見的(de)(de)大氣光(guang)學現象。產(chan)生(sheng)幻日的(de)(de)卷(juan)云(yun)也(ye)可以在天(tian)(tian)頂(ding)附近產(chan)生(sheng)環天(tian)(tian)頂(ding)弧(hu),通常發生(sheng)于(yu)太陽在天(tian)(tian)空(kong)角(jiao)度較低時(shi)。
形(xing)成(cheng)環(huan)(huan)(huan)(huan)天頂(ding)(ding)(ding)(ding)弧(hu)的(de)太(tai)(tai)陽(yang)(yang)光(guang)會(hui)(hui)從(cong)(cong)冰晶的(de)平坦頂(ding)(ding)(ding)(ding)面進入,從(cong)(cong)其中一個(ge)棱鏡面射出。幾乎平行的(de)太(tai)(tai)陽(yang)(yang)光(guang)經過角(jiao)(jiao)度(du)90°的(de)棱鏡折射后將是寬(kuan)色(se)分離(li)和(he)較單純的(de)顏色(se)。環(huan)(huan)(huan)(huan)天頂(ding)(ding)(ding)(ding)弧(hu)只能在(zai)太(tai)(tai)陽(yang)(yang)的(de)仰角(jiao)(jiao)低于32.2°時(shi)形(xing)成(cheng)。當(dang)太(tai)(tai)陽(yang)(yang)的(de)仰角(jiao)(jiao)在(zai)22°時(shi)的(de)環(huan)(huan)(huan)(huan)天頂(ding)(ding)(ding)(ding)弧(hu)最明亮(liang),因(yin)(yin)為這時(shi)的(de)太(tai)(tai)陽(yang)(yang)光(guang)進入和(he)離(li)開冰晶時(shi)的(de)偏(pian)向角(jiao)(jiao)最小;這時(shi)形(xing)成(cheng)的(de)環(huan)(huan)(huan)(huan)天頂(ding)(ding)(ding)(ding)弧(hu)長度(du)22°,寬(kuan)度(du)3°。環(huan)(huan)(huan)(huan)天頂(ding)(ding)(ding)(ding)弧(hu)的(de)長度(du)因(yin)(yin)為太(tai)(tai)陽(yang)(yang)仰角(jiao)(jiao)的(de)不同而在(zai)32.2°和(he)0°之間變化,并且在(zai)這兩個(ge)極值(zhi)的(de)環(huan)(huan)(huan)(huan)天頂(ding)(ding)(ding)(ding)弧(hu)亮(liang)度(du)會(hui)(hui)非常微弱。當(dang)太(tai)(tai)陽(yang)(yang)的(de)仰角(jiao)(jiao)高于32.2°,光(guang)線會(hui)(hui)從(cong)(cong)冰晶的(de)底部離(li)開,會(hui)(hui)形(xing)成(cheng)幾乎只有(you)單色(se)的(de)幻日環(huan)(huan)(huan)(huan)。
“環天(tian)頂(ding)(ding)弧”(Circumzenithal Arc)形(xing)成(cheng)于和(he)太(tai)陽(yang)同一側的天(tian)空,并且以不超過四(si)分之一圓(yuan)周的長(chang)度環繞天(tian)頂(ding)(ding)。當天(tian)晴云(yun)(yun)(yun)淡,陽(yang)光(guang)(guang)以一定的角(jiao)度,照射在高空云(yun)(yun)(yun)層中(zhong)的細小(xiao)冰(bing)晶(jing)上形(xing)成(cheng)折射,才會出(chu)現這個(ge)現象。同時,云(yun)(yun)(yun)層中(zhong)的冰(bing)晶(jing)表(biao)面一定得是彎曲的,而且顆粒比鹽粒還要(yao)小(xiao)。當太(tai)陽(yang)處于云(yun)(yun)(yun)層下方,日(ri)光(guang)(guang)與冰(bing)晶(jing)扁(bian)平(ping)面相接觸時,光(guang)(guang)線在每個(ge)晶(jing)體內都(dou)發生了彎曲,就會折射出(chu)彩虹特有的七色光(guang)(guang)譜。由于卷(juan)云(yun)(yun)(yun)所含的冰(bing)晶(jing)粒子不斷移動位置(zhi),因此環天(tian)頂(ding)(ding)弧現象出(chu)現的時間非常短暫。
環地平弧
日承對弧