The Brocken Spectre, here seen in Poland, is an optical phenomenon in which the observer’s shadow appears to be magnified on clouds or fog below. The Spectre can be observed from mountaintops when the sun is low and behind you, and there’s dense fog or clouds below. It is often accompanied by a glory, a rainbow-like halo that can also be observed when one is between the sun and a layer of clouds, and the movement of the clouds plus the apparent magnification can give the impression of a supernaturally tall ghost being walking the mountain.
The phenomenon is named for Brocken, also known as Blocksberg, a mountain peak in northern Germany long associated with witches and devils in local lore and literature. Another place to see it is the Scottish mountain Ben MacDhui, a frequently fog-shrouded peak where legend has it an unusually tall “Grey Man” resides. It isn’t hard to image how a lone mountaineer—halfway lost and hearing his own footsteps oddly distorted in the mist—could conjure up mythical beings when faced with a ghostly giant in the distance.