美国俄亥俄州上空的环水平弧虹
影像来源与版权:Todd Sladoje

说明:为何云会有其他颜色呢?19上图此例的原因是:远方卷云内的小冰晶,就像漂浮的棱镜把阳光散成虹彩。19而这种与地平面平行的环水平弧虹,因其形似火焰故亦称为火彩虹。19要看到这种环水平弧虹,天空有卷云的地方,当时太阳在天空的仰角要恰好是58度。19此外,卷云内要有无数水平排列的扁平六角形冰晶,以相同的方式一起折射阳光。19因此,环水平弧虹极为罕见。19上面这则透过偏光19镜记录的环水平弧虹,2009年摄于美国俄亥俄州的都柏林镇。(cirrus clouds 卷云;fire rainbow 火彩虹;circumhorizon arc 环水平弧虹)
翻译:“每日一天文图”成大物理分站

A Circumhorizontal Arc Over Ohio
Image Credit & Copyright: Todd Sladoje

Explanation: Why would clouds appear to be different colors? The reason here is that ice crystals in distant cirrus clouds are acting like little floating prisms. Sometimes known as a fire rainbow for its flame-like appearance, a circumhorizon arc lies 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 are present. Furthermore, 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 quite unusual to see. This circumhorizon display was photographed through a polarized lens above Dublin, Ohio in 2009.
Astronomy Picture of the Day 05/19/2019