Irisierende Wolken

Sometimes you can observe that certain parts of clouds exhibit iridescent colors. Such iridescence occurs mostly in clouds that form quickly (e.g., Altocumulus lenticularis). Especially the edges of the clouds show a purplish-red, blue, and green color. This phenomenon is closely related to coronas. Here, too, the colors arise from light diffraction. The water droplets that cause the iridescence are very small. Small droplets create very large coronas with wide sections of the same color. For this reason, large areas of the cloud exhibit the same color. The other colors on the iridescent cloud are less due to different distances from the sun, but rather due to the different droplet sizes. Different sizes of droplets create different coronas, so the color is different despite the same distance from the sun.  As results of continuous observations of atmospheric phenomena show, 12 percent of iridescent clouds are observed in cirrocumulus. This type of cloud consists mostly of ice crystals and to a lesser extent of freezing water droplets. Especially in cirrocumulus, iridescence is often observed at a solar distance greater than 30°, which almost excludes diffraction as the cause. Therefore, the latest theories assume that the colors are either caused by interference of rays reflected on the front and back surfaces of the thinnest ice plates or by interference of rays, some of which pass directly through the cloud layer while others are reflected one or more times within the layer.