A Cirrus Cloud Glory

On 20 January 1995, an apparent cirrus cloud glory was photographed over central Utah from an airline passing through the 10.7 km MSL level as it began its descent into Salt Lake City International Airport. The cirrus cloud layer was geographically widespread and optically thin enough to view the snow-covered Wasatch Mountain Plateau beneath the cloud. The glory was easily visible surrounding the shadow of the aircraft, ahead of a short contrail. Photographic analysis shows a single reddish-brown ring of ~2.1° radius surrounding a central bright spot, although a vague second ring appeared visually to have been present over the brief (~3-min) observation period. The presence of a glory can only be accounted for through the interference effects of the backscattered light rays for particles that are spherical in shape. Coincident Salt Lake City radiosonde data show a cirrus cloud top temperature in the -50° to -55°C range. We are unable to evoke the presence of other, more exotic cloud forming substances at the indicated cloud temperature to explain this phenomenon. Hence, employing Mie theory scattering simulations, we conclude that this apparently unique atmospheric optical observation provides evidence for the occurrence of a population of ~20 μm diameter spherical ice particles. This size is consistent with those derived from cirrus cloud top corona displays, although in such cases unusually high linear depolarization ratios suggest the presence of complicated particle shapes. The lack of vivid colorization in the glory implies that the particle size distribution may have been broader than those of typical cloud droplet populations producing glories, or that the ice spheres were somewhat irregular in surface complexity. The brief appearance of this cirrus cloud glory, and its location over mountainous terrain, suggests that the cloud generating mechanism was a mountain wave cloud at about the cirrus cloud top position.