A spectre of the Brocken occurs when the observer casts his or her shadow on a wall of fog. The wall of fog can be compared with a screen in a cinema, where the shadow is projected on, and magnified several times. But contrary to the screen, fog has no smooth (real) surface, so that it causes three-dimensional pictures which move in a spooky way by the waving of the fog, although the observer does not move at all.
Sometimes there forms a colourful, annular optical phenomenon around the "head" of the spectre of the Brocken, which is called a glory. This glory is caused by backward scattering and diffraction of the sunrays by the very small fog droplets. The rays of light become diverged by the small round droplets into a pattern of concentric circles.
Da sich die Glorie um den Gegenpunkt der Sonne (in diesem Fall um den Schatten des Berges, Gebäudes, Flugzeuges, Beobachters, etc) bildet, ist sie fast ausschließlich von erhöhten Standpunkten aus beobachtbar.
The larger the distance between observer and the fog is, the bigger and more colourful the glory becomes, but the spectre of the Brocken becomes smaller and fainter. But on the other hand, another phenomenon often becomes visible: the fog bow.
The first detailed description of spectre of the Brocken and glory was made by the famous Spanish scientist and captain Ulloa, who crossed the Andes in 1735, together with the French scholar Bougner.
The place of the term "Ring of Ulloa", which was internationally used until the end of the 18th century, was taken over by Goethe's "Theory of Colours" which he also made optical and meteorological studies for on Mt. Brocken. Thus the term "Spectre of the Brocken" made Mt.Brocken known in meteorological bibliography in the whole world. Due to its eccentric position, Mt.Brocken does not only have very thick fog (a visibility of just a few metres is not rare there!), but as it also has more than 300 foggy days a year, it is also the point where you have the best opportunities to meet the spectre of the Brocken.