美国西弗吉尼亚州上空的火彩虹

2021年8月30日

A Fire Rainbow over West Virginia
Image Credit: Christa Harbig

Explanation: What’s happening to this cloud? Ice crystals in a distant cirrus cloud are acting like little floating prisms. Known informally as a fire rainbow for its flame-like appearance, a circumhorizon arc appears parallel to the horizon. For a circumhorizontal arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds present below — in this case cirrus fibrates. The numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight in a collectively similar manner. Therefore, circumhorizontal arcs are somewhat unusual to see. The featured fire rainbow was photographed earlier this month near North Fork Mountain in West Virginia, USA.

Tomorrow’s picture: true moon blue

美国西弗吉尼亚州上空的火彩虹
影像提供: Christa Harbig

说明: 这朵云怎么啦？它是远处卷云里的冰晶，行为像漂浮的小三稜鏡所产生的景观。这种景观因形似火焰而有火彩虹的称号，较文邹邹的名称则为平行于地平线的日承(circumhorizon arc)。要看到日承，太阳在天空的仰角必须大于58度，而且它的下方一定要有卷云，在此例里为纤维状卷云。此时，卷云里难以数计的扁平六角形冰晶必须平行于地表排列，才能以相似的模式共同折射阳光。因此，火彩虹算是很罕见。上面这幅记录火彩虹的主题影像，是在这个月稍早摄于美国西弗吉尼亚州的北福克山脉附近

明日的图片: true moon blue