Coronas and Iridescence in Mountain Wave Clouds Over Northeastern Colorado

Abstract

Scattering of sunlight or moonlight by cloud particles can generate colorful optical patterns that are both scientifically and aesthetically interesting. Photographs of corona rings and iridescence are presented to demonstrate how cloud-particle distributions and meteorology combine to produce a wide variety of observed patterns. The photographs of coronas are analyzed using Fraunhofer diffraction theory to determine that these optical displays were generated by cloud particles with mean diameters ranging from 7.6 to 24.3 µm. All examples of coronas and iridescence presented in this paper were observed within mountain wave clouds along the steep lee side of the Rocky Mountains over northeastern Colorado. Such clouds, commonly observed both here and on the downstream side of many other prominent mountain ranges, tend to have small cloud particles with narrow particle-size distributions, conditions that lead to relatively frequent and vivid optical displays. The meteorology accompanying at least one-half of the displays presented here suggest that the wave cloud particles consisted of ice, whereas, at least until recently, it has been accepted that spherical liquid cloud droplets are primarily responsible for coronas and iridescence. Microphotographs of particles collected from the interior of similar mountain wave clouds show that such clouds can indeed contain quasi-spherical ice particles with effective diameters less than 25 µm, which provide a mechanism for the highquality optical displays to be generated within wave clouds at high altitudes with temperatures below ?36° to ?38°C. In fact, these quasi-spherical ice particles may be commonly associated with mountain wave clouds, thus suggesting that this type of ice particle may regularly produce coronas and iridescence.