Coronas and Iridescence in Mountain Wave Clouds over Northeastern Colorado

OCTOBER 2003 AMERICAN METEOROLOGICAL SOCIETY | T he interplay of sunlight, moonlight, or anthropogenic light sources with cloud and precipitation particles can produce a wide variety of optical phenomena, some with dazzling arcs and colors, in the sky around us. These naturally occurring patterns of redistributed light, often referred to generally as atmospheric optics, represent an especially aesthetic component of the field of meteorology. A comprehensive description of natural optical phenomena and their causative mechanisms are provided by Minnaert (1954), Tricker (1970), Greenler (1980), Meinel and Meinel (1983), Minnaert (1992), Tape (1994), and Lynch and Livingston (2001), among others. Cloud and precipitation particles that produce optical displays are composed of either ice crystals or liquid water drops. Ice crystals are responsible for an incredibly rich palette of halos, arcs, and pillars arising largely from the refraction and/or reflection of light by these crystals. Liquid water particles also produce a variety of optical phenomena, though fewer in number than those arising from ice crystals. Nevertheless, this latter class of displays often exhibits striking color. The most commonly known of these is the rainbow, formed by the refraction and reflection of light by raindrops, creating colored circles of light centered on the point directly opposite the sun or other light source. Equally beautiful are the colorful diffraction patterns caused by light passing through clouds composed of small liquid or ice particles. These patterns are typically referred to as coronas, iridescence, and glories, though this discussion will not include the glories since they are not observed regularly from the ground. Coronas and iridescence are often quite colorful, and they occur rather frequently. However, many are not aware of these diffraction displays during daylight hours because the colors usually remain close to the blinding rays of the sun. CORONAS AND IRIDESCENCE IN MOUNTAIN WAVE CLOUDS OVER NORTHEASTERN COLORADO