March 11, 2003
You are a way for the cosmos to know itself, Carl Sagan once said. On your birthday, the cosmos got to know itself a little better.
Iridescent Clouds
Birthday · March 11, 2003
A little universe, just for you
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APOD published March 11, 2003. Credit: Arne Danielsen. A publication-date match, not a photo taken on your date. Independent of NASA.
Why would clouds appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract 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 far from the Sun to exhibit striking colors. Pictured above, iridescent clouds were photographed over Norway two months ago.
Image credit: Arne Danielsen