Look down from a mountain or an aircraft onto cloud or fog with the sun behind you, and you may see a set of colored rings on the cloud below. They sit around the head of your shadow. Somebody standing beside you sees their own set, around their own head, and not yours. Why is it fixed on the observer?
Geometry, not coincidence. A appears when light hits the small making up cloud or fog, separating into colors as in a rainbow. The way that light bounces inside the droplets returns it as a small series of circles rather than one wide arc. The rings land at the head of your shadow because that is the point directly opposite the light source from where you are standing.
A glory is an that casts what looks like a rainbow halo around the head of an observer’s shadow. The conditions are usually best in high places such as mountains and tall buildings, and those are also where elongated shadows called form, so the two tend to appear together.
The optics start the same way a rainbow does. Light hits the small water droplets that make up cloud or fog and separates into the colors that compose it. What differs is what the light does inside the droplets: the way it bounces around returns a small series of circles rather than one wide arc.
Height matters for a specific reason. The water droplets have to be level with the light source, and the light source has to be below you, so that the halo lands at the head of your shadow. Satisfy that and the rings follow you; fail it and there is nothing to see.
The Brocken spectre has the same requirements and a stranger effect. Casting your shadow down into cloud or fog, the light exaggerates your figure until you seem much larger and longer-limbed, and movement in the droplets can make the shape appear to stir by itself. A ringed halo around an enormous moving silhouette fueled superstition for centuries, which is a reasonable response to it.
wat. The rings are not on the cloud. They are on the line between the sun, your head and the cloud, which nobody else is standing on.
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A rainbow belongs to nobody in particular. A glory is addressed to a single viewpoint, which is a useful demonstration that these effects are directions rather than objects.
How do we know?
The National Weather Service describes glories, the conditions they need, and their frequent companion the Brocken spectre, in its guide to unusual atmospheric phenomena.
What is still unknown
An educational page rather than a treatment of the optics. It explains that the light bounces around inside the droplets to give circles instead of an arc, without going into why that produces rings at those particular angles.
Learn something?
Where to next
Why is a rainbow in fog white?optics
Why do sunbeams fan out from a point in the clouds?optics
What was the band of rainbow colors over Spokane with no rain anywhere?light