A man has captured a Brocken Spectre on video while walking in the hills of the UK. Chris Randall, who calls himself an ultramarathon runner and snowboarder, shared the footage with his Twitter audience, calling it "creepy" to encounter, and initially mistaking it for "someone else".

There is a logical explanation, you'll be relieved to hear, that doesn't involve spooky ghost clouds. Despite this, as one Twitter user put it, "even with the explanation, if I saw that on a fell I’d be back in bed, sheets up to my neck within minutes".

The specters have been seen for millennia and may explain sightings of "Dark Watchers" that have been reported for centuries in California. The phenomenon can even happen with planes.

First up, the explanation bit. Brocken Spectres – named after the Brocken mountain where they were first described by German scientist Johann Silberschlag – are created by us on misty days. 

When the Sun is low and the conditions are right, a shadow is cast by the observer onto the mist, making it appear as if a tall, shadowy figure is watching them from nearby. The water droplets that make up the mist can shift around, causing a disorientating effect, as though the shadow is moving, sometimes towards the observer. So, people are literally being scared by their own shadows.

Hikers in the Santa Lucia Mountains of California have reported the presence of "Dark Watchers" for centuries. From the 1700s, there have been accounts of walkers seeing tall, featureless silhouettes on the horizon, motionlessly watching them on their journey. Hikers in Big Sur have described feeling as though they are being watched, before turning around and catching a short glimpse of an unknown figure that vanishes shortly after they are seen. 

The figures were famously mentioned in a poem by Robinson Jeffers in 1937, as well as in John Steinbeck's short story Flight the following year. In Steinbeck's story, the protagonist flees to the mountains after killing a man in a fight. Before he leaves, his mother warns him "when thou comest to the high mountains, if thou seest any of the dark watching men, go not near to them nor try to speak to them.”

Lo and behold, on top of the mountains "he saw a black figure for a moment; but he looked quickly away, for it was one of the dark watchers."

In Jeffer's poem, the figures are described as almost, but not quite human:

"He thought it might be one of the watchers, who are often seen in this length of coast-range, forms that look human to human eyes, but certainly are not human. They come from behind ridges to watch. But when he approached it he recognized the shabby clothes and pale hair and even the averted forehead and concave line from the eye to the jaw, so that he was not surprised when the figure turning toward him in the quiet twilight showed his own face. Then it melted and merged into the shadows beyond it."

It would be easy to dismiss the phenomenon as an old myth, were it not for suspiciously similar reports around the world. In Scotland, on the peak of Ben Macdui, a similar figure has been seen since 1891. Am Fear Liath Mòr – Scottish Gaelic for '"Big Grey Man" – has been described in several dramatic encounters over the centuries. In the first recorded description, the poet James Hogg described it as "at least thirty feet high, and equally proportioned, and very near me. I was actually struck powerless with astonishment and terror".

Though many explanations have been proposed for the sightings – from pareidolia to hallucinations brought on by exhaustion – the most convincing is that people are seeing Brocken specters made by their own shadows.