From AMS Glossary
An observer's shadow cast upon a cloud.
This usually happens when the observer is on a mountain top or ridge and a low sun casts the shadow onto a fog or cloud in the valley below. Although the shadow is essentially the same size as the person, the observer sometimes gains the impression that it is gigantic. This is likely the result of a comparison between the nearby shadow and distant objects glimpsed through the cloud. It was named after early observations made by climbers on the Brocken, a peak in the Harz Mountains of Germany.