Brocken Spectres and Fog Bows explained
Posted on Mar 07, 2014 by Tim Francis
No, Brocken Spectres and Fog Bows are not a race of alien beings from a Doctor Who episode, incase you’re wondering.
Let Wilderness Guide, Tim Francis, enlighten you; Brocken Spectres and Fog Bows explained.
The Brocken Spectre
The wind sculpted summit ridge lay before us. Snow and ice sharpened to a defined crisp edge that dropped vertically into the mist below. Our party moved with purpose and care towards the reward of the summit cairn. The cloud slowly cleared from the south as we approached the final few meters to reveal a panorama of awe-inspiring mountain scenery. Then a surprise. There to greet us in a scene of psychedelic wonder we were greeted by the dark image of a giant waving hippie with long shadowy flares and surrounded by a multi coloured halo!!
This sounds like an extract from a supernatural climbing novel but is in fact a true account of our summit walk on Ladhar Bheinn, during our Knoydart walking trip just last weekend. The Psychedelic hippie a natural phenomenon called a ‘Brocken Spectre’.
Winter is a great time to be in our wild mountains and over the last few years I’ve witnessed Brocken Spectres and Fogbows (white rainbows) during this time. Folk tales abound with stories of strange ghostlike figures in our mountains. The most famous of them must be the Big Grey Man of Ben MacDhui.
What is a Brocken Spectre?
A Brocken Spectre isn’t actually a giant waving hippie. The term ‘Brocken Spectre’ is so named because of sightings on the Brocken, the highest peak of Germany’s Harz Mountains.
This optical effect appears when sunlight, coming from directly behind the climber, falls onto a layer of cloud droplets. Since the observer needs to be looking down onto the cloud layer, for the Sun to be shining from directly behind him along his line of vision; Brocken Spectres tend to only occur in high places. The observer’s shadow, hugely distorted by the effects of perspective, appears in the middle of a halo of colours, which are officially known as a ‘glory’. All shadows converge towards the antisolar point where the glory also shines.
What are Fog Bows?
Fog Bows are similar to rainbows except that they appear as a bow in fog rather than rain. Fog water droplets are much smaller than those of rain. A fog bow has only very weak colours, with a red outer edge and bluish inner edge. When the droplets are very small, fog bows appear white, and are therefore sometimes called white rainbows. This minute water droplet size is such that the wavelength of light becomes important. Diffraction smears out colours that would be created by larger rainbow water drops.
So next time that you are out on the mountains, surrounded by mist with a hint of the sun. Take time to look not just for views but maybe also a friendly wave from a flower power friend.
Have you seen either of these phenomenon? Please tell us about it in the comments below.
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