A few years ago we were winter walking on the Mamore Range south of Ben Nevis when we spotted our first Brocken spectre, which was being cast onto the southern slopes of Ben Nevis.
What is a Brocken Spectre? High altitude travellers such as mountaineers and aircraft passengers have often reported seeing shadows on clouds surrounded by concentric rainbow-like rings of colours. Such phenomena are known as glories – or in the mountaineering community – Brocken spectre.
Where does this name come from? It is alleged that the term “Brocken spectre” is named after an old story (folklore?) emanating from the mountain of Brocken in Germany. Apparently, a startled climber fell to his death on seeing his own shadowy figure accompanied by its “glory” ring. This shadowy ghost-like figure, or spectre, has been coined to describe this phenomenon.
So how does a Brocken spectre occur? There are two requirements that must be met in order to see one’s own Brocken spectre. These are that:
- There must be many water droplets in the region where the glory is to appear (the size of the water droplets is probably also important);
- You must be looking directly away from the sun (anti-solar direction).
The first condition occurs when there is mist, fog or cloud in the atmosphere. In the mountains, the second condition best occurs when the sun is low in order to cast one’s shadow a long distance and hence onto any potential mist. (Compare, for example, when the sun is overhead, then your shadow is effectively at your feet!)
I haven’t yet worked out if the physical principal underlying the phenomenon is that of diffraction or refraction. Instinctively I’d say it was refraction, but several web sites say that it is caused by diffraction (answers on a postcard please!) One explanation I’ve heard states that sunlight penetrates individual water droplets and reflects off their backsides. Some of the light heads back towards the sun and interferes with the direct sunlight causing diffraction patterns(?) that are essentially the circular zones of colour. Red is always seen on the outside of the pattern.
(Incidentally, you only see your own glory, which will be centred around the head of your shadow. A partner, only a few feet away, will not see yours – nor you theirs.)