The Brocken Spectre
Most air travelers have observed glories in the vicinity of their aircraft's shadow on the clouds below. The glory is a part of a phenomenon familiar to mountaineers who know it as the Brocken Spectre.
According to legend, the name came from the unhappy circumstance of a climber on northern Germany's highest mountain, the Brocken. Working his way across a narrow precipice, the climber was startled by the sudden appearance in the nearby mists of a human figure with a ring of light around its head. Frightened, the climber fell to his death. If this event really took place, it was the climber's own shadow that he saw, and the ring of light was his own "glory" ring.
Two requirements must be met if one is to see her or his own glory. One must look exactly in the anti-solar direction and there must be many water droplets in the region where the glory is to appear. Since the shadow of one's own head appears only in the anti-solar direction, it makes sense that the glory ring will always be in the vicinity of the head's shadow.
Glories are common in high-latitude regions where the low sun angle allows a person to easily come between the sun and a fog or cloud bank. But glories are most easily seen when one is riding on the shadow side of an aircraft above the clouds. The clouds below provide the necessary water droplets.
The glory ring is caused by the phenomenon called diffraction. Sunlight penetrates individual water droplets, be they in rain, fog or clouds, and reflects off the back sides of the droplets. Some of the light emerges from the droplet to come back essentially toward the sun. But the sunlight coming back toward the sun from the different raindrops interferes with itself to create circular zones of darkness and brightness. The diameter of the zone for each color of light is different. Consequently, the glory an airplane passenger sees always is red on the outside, just as is a primary rainbow. If more than one glory is seen, the color pattern repeats, red always being outermost.