This occurrence is known as a Brocken spectre, named after a peak in the Harz Mountains where it is often sighted. It usually appears in conjunction with the glory and is caused when the sun casts an observer’s apparently magnified shadow onto the bank of clouds opposite. The size distortion is an optical illusion resulting from the clouds’ obscuration of familiar reference points, and its effect is deeply uncanny. In ‘Nature’s Magic Lantern’, Hogg describes how, when he first witnessed his magnified shadow in this way, it “at first appeared to my affrighted imagination as the enemy of mankind”.
This video clip shows the otherworldliness of the Brocken spectre.