(The stratosphere's air is so dry, clouds can only form when temperatures are extremely cold, as in -100 F cold!) Given their high altitude, these clouds actually receive sunlight from below the horizon, which they reflect to the ground at dawn and just after dusk. Taking their name from their "mother of pearl"-like appearance, nacreous clouds are rare clouds that only form in the extreme cold of the polar winter, high up in Earth's stratosphere. In fact, you'll need to travel up to the world's farthest polar regions and visit the Arctic (or Antarctica in the Southern Hemisphere). To see a nacreous or polar stratospheric cloud, you'll have to do more than simply look up.
Arcs will be located far below the Sun or Moon (whereas cloud iridescence can be found anywhere in the sky), and its colors will be arranged in a horizontal band with red on top (in iridescence, the colors are more random in sequence and shape).ĭAVID HAY JONES/SCIENCE PHOTO LIBRARY / Getty Images How can you tell a circumhorizontal arc from an iridescent cloud? Pay close attention to two things: position in the sky and color arrangement. While they may not be as ahh-inducing as the rainbow, a circumhorizontal arcs do have a one-up on their multi-colored cousins: their colors are often much more vivid. (To get an arc rather than a sun dog, the Sun or Moon must be very high in the sky at elevations of 58 degrees or greater.) Part of the ice halo family, they form when sunlight (or moonlight) is refracted off of plate-shaped ice crystals in cirrus or cirrostratus clouds. They look like large, brightly-colored bands that run parallel to the horizon. Often called "fire rainbows," circumhorizontal arcs aren't clouds per se, but their occurrence in the sky does cause clouds to appear multi-colored.
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