Brocken Spectre

The ghostly apparition was encountered by Peter O'Toole on the summit of Slieve League, South Donegal, Ireland.

Images ©Peter O'Toole.
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The Brocken is the highest peak of Germany's Harz Mountains. Although only 1141m high, conditions on it are more extreme and like those on much higher peaks. It has a mean temperature of only 2.9 celsius and is said to be mist shrouded for up to 300 days each year.

The mist causes the legend. Early climbers would see a ghostly and apparently huge figure loom out of the mist. Its head was sometimes wreathed in an ethereal glow and even coloured rings. The "Spectre of the Brocken" was born.

The Brocken Spectre is a combination of two optical effects, one elementary and the other very complicated indeed.

The huge figure is the observer's shadow projected through the mist as the sun breaks through. It sometimes appears huge because the misty landscape provides few references with which to judge size.

The glow and sometimes rings around the Spectre's head is a glory.

The small water droplets of the mist diffract sunlight and scatter it almost directly backwards to form the glow. Although the glory is now easily simulated in a matter of seconds by a laptop, the actual physical mechanism of the scattering is not easy to visualize or understand.