(n.) an eerily elongated shadow cast by someone stood above the cloud line
A Brocken spectre, or Brockengespenst, is formed when a person stands on a slope or at a height with the sun behind them, in misty conditions or amidst low-lying cloud. Those conditions produce a curious meteorological phenomenon, in which they appear to cast an eerie elongated shadow.
It is the precise position of the sun and the angle of the slope that conspire to cast the observer’s shadow downwards, thereby elongating and distorting it—while the fact that the shadow is partly cast onto the droplets of water vapour in the misty air, at varying distances from the observer, tricks the brain’s interpretation of depth perception, and ultimately gives the shadow a magnified, misshapen appearance.
Add to that the fact that this morphed shadow can appear to flicker or even move as the clouds onto which it is cast drift past, and all told a Brocken spectre can prove a fairly disconcerting occurrence.
First described in the eighteenth century, Brocken spectres like these take their name from a snow-capped 3,700ft peak in central Germany called the Brocken, whose location, orientation, and misty microclimate all conspire it a prime spot for experiencing precisely this kind of phenomenon.