This is the fourth post in a series on optical phenomena in Antarctica. This post covers cloud iridescence, one of the most elegant and beautiful optical effects I’ve seen.
Above: A golden cloud with iridescence, appearing as a rainbow streaking through the cloud. Photo taken from McMurdo town outside of building 155.
Cloud iridescence is when a cloud appears to have rainbow colors in it. It happens when there is a relatively thin layer of very uniform water droplets or ice crystals. It is most common in altocumulus, cirrocumulus, lenticular and cirrus clouds, however it can happen in any thin cloud, such as one forming or dissipating. When the sun hits a very thin, uniform layer of droplets, the droplets diffract the light to give us rainbow colors. It is important that the droplets are small and consistent in size/shape throughout the cloud (otherwise the light is scattered more randomly and the colors become washed out). The colors are often most vivid near the sun, where diffraction effects are strongest. The effect is even more striking in the photo below, a cloud seen in the skies above Boulder, CO.
Above: Two photos of the same cloud above Boulder, CO, showing a remarkable display of cloud iridescence.
