Have you ever seen the edges of very thin clouds look like oil on water, like the inside of a sea shell, like a smudgy rainbow?
If you have, then you've seen cloud "iridescence."
It's actually quite rare. Iridescence is one of many optical phenomena that occur when sunlight interacts with cloud particles.
Follow through these images with their figure captions to understand this process by which Nature creates yet another wonder in the sky!
Figure 1: Very thin and very high clouds are the first ingredient needed for iridescence. In this satellite image, high clouds sit south and west of Tucson.
Figure 2: Here's a photo by Ed Erbeck of iridescence. See the "pearl-like" effect in the clouds?"
Figure 3: The clouds must again be VERY thin. Also, the particles in the clouds must all be roughly the same size and have roughly the same distance between them.
Figure 4: Some incoming sunlight of course hits the particles. But, this is NOT what causes iridescence.
Figure 5: Iridescence happens when sunlight passes through the openings between the particles. It's like they dodged the particles, but now they are bent, not moving in a straight line.
Figure 6: This image tells the story. Picture waves coming in from left to right. Then they hit the red-orange barrier. The waves are ABSORBED by the red-orange barrier. BUT, that barrier has holes in it. Some of the wave passes between the barriers, through the gaps. But the wave is now bent. Since it isn't going in a straight line any longer, it bumps into waves getting through other gaps. The "gaps" in this theoretical image represent the space between those particles in Figure 2.
Figure 7: Since sunlight is made up of all colors of the rainbow, when the bent waves bump into one another, instead of just being brighter and darker spots, they become color distortions. This creates the iridescence. Notice in this image that it works in other things besides clouds, too. The image in this figure is of a dandelion bloom. See the iridescence?
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