2020 January 15

Iridescent Clouds over Sweden
Image Credit & Copyright:
Goran Strand

Explanation: Why would these clouds multi-colored? A relatively rare phenomenon in clouds known as iridescence can bring up unusual colors vividly or even a whole spectrum of colors simultaneously. These polar stratospheric clouds clouds, also known as nacreous and mother-of-pearl clouds, are formed of small water droplets of nearly uniform size. When the Sun is in the right position and, typically, hidden from direct view, these thin clouds can be seen significantly diffracting sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too angularly far from the Sun to exhibit striking colors. The featured image and an accompanying video were taken late last year over Ostersund, Sweden.

瑞典上空的彩云
影像提供与版权:
Goran Strand

说明: 为何这朵云的色彩会如此缤纷呢?一种相对少见的彩云现象,能够让云带着不寻常的鲜明色彩或七彩并现。影像中这种又名为贝母云或珠光云的极地平流层云,是由大小几乎均一的小水滴所组成。而当太阳位在适当的位置,同时又被厚云遮住时,这种薄云会大量且同相的绕射阳光,把不同色彩的阳光折到不同的方向。所以人们就会看见不同颜色的光,来自稍微不同的区域。许多云刚开始时,常有一些均匀且可以产生彩云的区域,不过很快的就长得太厚、混合过度、或离太阳的角度太远,以至于无法造成这种鲜明的色彩。上面这张主题影像及附随的影片,是在去年底摄于瑞典.厄斯特松德市的上空。

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