Wednesday, November 22, 2006

Glory in the Skies

A Multi-ring glory surrounds an aircraft's shadow. by Philip Laven

A glory is an optical phenomenon produced by light backscattered (a combination of diffraction, reflection and refraction) towards its source by a cloud of uniformly-sized water droplets. A glory has multiple colored rings. The angular size is much smaller than a rainbow, about 5° to 20°, depending on the size of the droplets.

Since it is seen in the direction opposite the sun, it is most commonly observed when on a mountain above the clouds, or while airborne, with the glory surrounding the airplane's shadow on clouds.

If you look toward the antisolar point, the place in the clouds directly opposite the sun, if the aircraft is low enough, you will find the shadow of the plane. Surrounding the shadow is the glory, a bright white glow surrounded by one or more shimmering rings of color.

These rings are formed when light is scattered backwards by individual water droplets in the cloud. The more uniform the size of the droplets, the more rings you will see. They swell and contract as you travel over clouds with smaller or larger droplets.

In China, this phenomenon is called Buddha's light (佛光). It was often observed on cloud-shrouded high mountains, such as Huangshan Mountains and Mount Emei. Records of the phenomenon at Mount Emei date back to A.D. 63. The colorful halo always surrounds the observer's own shadow, and thus was often taken to show the observer's personal enlightenment (associated with Buddha or something divine) until modern science explained the optics behind the phenomenon.

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