A glory is an atmospheric optical phenomenon consisting of a series of concentric coloured rings (similar to a rainbow but much smaller) appearing around the antisolar point — the point diametrically opposite the Sun as seen from the observer. It is produced by the backscattering of sunlight in the water droplets of a cloud or fog.
Optical mechanism
Unlike a rainbow (caused by refraction and internal reflection in large drops), the glory is produced by a complex mechanism involving surface wave diffraction around small, uniform droplets (typically 5-20 µm diameter). Light enters the drop, undergoes internal reflections, and emerges in the reverse direction. Interference between waves travelling around the drop's surface on both sides produces the coloured rings.
It is frequently observed from aircraft as a set of bright rings around the shadow of the plane projected onto clouds (the "pilot's halo"). In mountains, it forms part of the Brocken spectre, surrounding the observer's giant shadow. The diameter of the rings is inversely proportional to the droplet size.
In Spain, glories are frequent in the mountains when clouds fill the valleys and the sun illuminates from behind: the Pyrenees, Picos de Europa, and all mountain ranges with morning thermal inversions are privileged locations for observation. It is an ephemeral and delicate phenomenon requiring precise conditions of illumination and uniform droplet size. See also: Brocken spectre, Belt of Venus.