Scottish Mountaineering Club Journal Volume 3 Number 2

Notes on Brocken spectre

by R. T. Omond, Superintendent, Ben Nevis Observatory

ATMOSPHERIC optical phenomena may be classed in two groups - those seen looking towards the source of light (sun or moon) and concentric with it, and those seen looking away from the light and concentric with the shadow of the observer. This classification corresponds to the names parhelia and anthelia; but it is well to keep the specific names of the various phenomena seen in each direction definite and distinct. The following are those most commonly seen:-

I. Parhelia, or concentric with sun.

Halo. - Large circles, 22º to 90º radius, which may have radial lines crossing them, tangential arcs, or mock suns, white or coloured.

Coronae. - Small circles, not more than 10º radius, always coloured, with the red outside each set of colours; there may be one, two, three, or four sets of spectrum colours.

II. Anthelia, or concentric with shadow.

Rainbows. - Formed on drops of water, the primary or inner bow about 40º radius, and having red outside. The secondary about 52º, and having red inside; sometimes spurious bows, repetitions of the primary, may be seen inside it.

Fog-bows. - Similar to rainbows, but formed on a bank of cloud or fog, not on raindrops. The radii vary slightly - say 35º to 55º - and the order of colours is often inverted, and are always fainter and more diffused, both in colour and outline, than rainbows.

Glories. - Circles, generally coloured, and then always with red outside, formed round the shadow of observer, As many as five sets of spectrum colours have been seen, the radius of the outer never exceeding 12º, and the colours filling practically the whole space from the shadow outwards, thus distinguishing them from fog-bows. These are occasionally seen at the same time as glories, the latter forming a mass of colour round the shadow, and the fog-bow a wide arc quite distinct and far away from it; comparable to the nave and rim of a carriage-wheel of which the spokes are invisible.

[page 93]

The necessary condition for the formation of a glory is that the sun or moon shall be sufficiently bright to cast a sharp shadow, and that that of the observer shall fall on a bank of cloud or fog. Naturally this is most likely to happen when standing on an elevation, and when the sun is low; but glories have been seen on the low fog that lies only a foot or two thick over hollow ground. When standing at the edge of a cliff, with the sun high up behind and mist floating about below, the glory is formed on a definite part of the mist, its distance away - whether a few feet or many hundreds - is obvious, and the central shadow appears of its natural size. But when the sun is low, and the shadow is thrown on mist rising out of a valley alongside of the spectator, then a vivid imagination beholds the Brocken spectre in all its terrors. The shadow and glory are formed on the surface of the mist near the spectator, but he regards it as being at the furthest limit that he can see through the mist. Thus if there be only a thin wreath of fog through which a hill-top several miles off is visible, the shadow, really life-size at say ten yards off, becomes a giant on the far-away ridge. The above diagram represents a case with the sun near the horizon.

If two or more persons are present, each sees a glory on his own shadow, that being a diffraction effect dependent on the observer, the sun, and the fog; but he may see the shadows of his friends as well as his own. It is interesting, when the fog is near, to notice how the shadow of a person a few yards off forms a long dark tunnel running back into the fog.

[page 94]

All of the five classes of optical phenomena mentioned above have been seen both with sunlight and moonlight, and may even be produced with an artificial light; but the superior brightness of sunlight makes the solar cases more frequent and easier to observe, except in the case of haloes where the dazzling effect of the sun often prevents them being noticed.