A glory rainbow is a rare and striking atmospheric optical phenomenon, appearing as a series of concentric, colored rings. This ethereal display, often resembling a halo, forms under precise conditions.
Understanding the Glory Phenomenon
A glory appears as circular, colorful rings centered around the observer’s shadow on a cloud or fog layer. The colors usually arrange with blue on the inside and red on the outside, appearing bright and pure when the water droplets are uniform in size. This optical effect results from sunlight interacting with tiny water droplets through backscattering, interference, and diffraction effects.
The formation of a glory differs from that of a traditional rainbow. Rainbows occur due to the refraction and reflection of light within larger raindrops, typically appearing opposite the sun. In contrast, glories involve light scattering backward from much smaller cloud or fog droplets, creating a smaller angular size, typically between 5° and 20°. The interaction of light waves within these tiny droplets is a complex process that generates the distinct colored rings.
The Unique Conditions for Their Rarity
The rarity of observing a glory stems from the specific atmospheric and geometric conditions required for its formation and visibility. These conditions must align simultaneously, which happens infrequently for ground-level observers.
One precise requirement is the size of the water droplets within the cloud or fog. For a distinct glory to form, these droplets must be spherical and fall within a very narrow and uniform size range, typically around 10 to 50 micrometers in diameter. If there is too much variation in droplet size, the colored rings become less distinct or may not appear at all, as larger droplets cause the glory to shrink and become less prominent.
The observer’s position is another precise factor. A glory is always centered on the antisolar point, directly opposite the sun from the observer’s perspective and aligning with their shadow. This means the light source, the observer, and the cloud or fog layer must be in a direct line, with the observer positioned between the light source and the cloud. This specific alignment necessitates that the observer typically be above the cloud or fog layer, such as when looking down from an airplane, a high mountain, or a tall building. While glories were once rare sightings for mountaineers, air travel has made them more commonly observed by passengers looking out of airplane windows, often appearing around the aircraft’s shadow on the clouds below.