瑞典上空的彩云

瑞典上空的彩云

Iridescent Clouds over Sweden
Image Credit: Goran Strand

Explanation: Why are 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 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 in 2019 over Ostersund, Sweden.

Tomorrow’s picture: red planet, blue planet

瑞典上空的彩云
影像提供: Goran Strand

说明: 为何这些云的色彩会如此缤纷呢？一种相对少见的彩云现象，可让云带着不寻常鲜明的色彩或变得所有色彩并现。影像中这种亦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，同时观察者又未直视太阳时，偶而可以见到这种薄云大量、同相的绕射阳光，把不同色彩的阳光折到不同的方向。因此人们就会看见不同颜色的光，来自稍微不同的区域。许多云刚开始形成时，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，以至于无法产生这种鲜明的色彩。上面这张主题影像及附随的影片，是在2019年底摄于瑞典的厄斯特松德市。

明日的图片: red planet, blue planet