The Brocken spectrum or mountain spectrum is a huge shadow of an observer, cast over the clouds, opposite the sun. The phenomenon can appear even with airplanes.
The name derives from Brocken, the highest peak of the Harz mountain range in Germany. As the peak is above cloud level and is a frequently misty area, conditions conducive to spectrum formation on a cloud layer are common.
It appears when the sun shines behind an observer standing on a ridge or somewhere in fog or fog. The observer's shadow is cast through the mist, often with a strange triangular shape. The apparent magnification of shadow size is an optical illusion. The shadow can sometimes move because of cloud layer movement or variations in cloud density.
The observer's head is often surrounded by a bright halo (glory), shaped like colored light rings that appear directly opposite the sun when sunlight is reflected by a cloud of uniformly sized water droplets. The effect is caused by the diffraction of visible light.