The Brocken spectre (or Brocken bow) is an atmospheric optical phenomenon in which the observer's shadow is projected, enormously magnified, onto a surface of fog or cloud situated below or in front, surrounded by a glory — concentric rings of rainbow colours. It takes its name from Mount Brocken in the Harz Mountains (Germany), where it was first described in 1780.
Optical mechanism
It occurs when the Sun is behind the observer and illuminates the fog or low cloud ahead. The shadow is projected onto the water droplets, and the backscattering of sunlight by the droplets produces the glory. The shadow appears enormous because it is projected onto a non-flat surface (the fog) at a distance, creating an exaggerated perspective effect. Movements of the fog can make the shadow appear to move and "dance".
Ideal conditions are: low-angle sun (sunrise or sunset), observer at an elevated position (mountain summit, cliff edge, aircraft), and fog or low clouds below or ahead. In Spain, it is common in the Pyrenees, Picos de Europa, Sierra Nevada, and Sierra de Guadarrama, especially at sunrise when valley clouds have not yet dissipated.
The Brocken spectre has generated numerous legends in different cultures: mountaineers who believed they saw giants or spirits in the fog. It is a purely optical phenomenon, completely harmless, and one of the most beautiful sights a mountain lover can witness. See also: glory, Belt of Venus.