Circular Rainbow seen from a Hill-top

Abstract

IN the Philosophical Magazine for January, 1884, p. 61, is an interesting article by Prof. Tyndall describing experiments made to produce circular rainbows by artificial light and artificial mist, his attention having been attracted to the subject by an observation made in the Alps on one occasion when the shadow of his body was projected at night time on to mist by a lamp behind him, and was seen to be surrounded by a luminous circle, or halo of light. I was so fortunate as to see lately identically the same effect produced with remarkable beauty and completeness in broad daylight from the summit of a Welsh hill. Staying last week for a couple of days at Pen-y-Gwryd, near Snowdon, in company with a friend, we walked one morning up the Glydr-Vach. The rain was steadily descending as we left the little inn, and the thick mist swathed the hill-sides in obliterating folds. Just as we reached the summit at noon a slight breeze thinned away the mist in front of the sun, and a burst of sunshine illuminated the hill-tops. Clambering on to the natural cairn which crowns the summit, we looked down into the valley, in which lies the small lake Llyn Idwal. Along the valley the wind drove masses of thin mist and scud, and on this we saw to our surprise the shadow of the summit with our own sharply-marked shadows projected on it. We waved our arms, and the mystic figures replied by waving theirs. Surrounding these immense shadowy figures we could see two concentric rainbows completely circular, the centre being the shadow of our heads. The colours of the inner rainbow were in the order of the primary bow, and the outer was a secondary and more faintly-tinted rainbow. During all this time the sun was shining brightly on our backs; when the wind cleared away the mist completely in the valley, the shadows and the rainbows vanished, but reappeared when fresh masses of vapour were blown into the line of our shadows. A very rough attempt at determining the angle subtended by the diameter of the primary bow seemed to show that it was much less than 90°, in fact not probably above 20°. This interesting appearance lasted only for a few minutes, as the wind drove up fresh mist in front of the sun, and the rainbow-circled phantoms disappeared. It would be interesting to know if any of your readers have ever observed a similar phenomenon. It has, I believe, been seen by balloonists when the altitude of the sun is great and a layer of mist and cloud lies beneath. Shadows thrown on mist are common; but this rainbow addition was new, not only to me, but to my friend, and his mountaineering experience has been very considerable.