Unusual Rainbow

Abstract

A RAINBOW after sunset is probably a somewhat unusual occurrence, but on the evening of September 11 I witnessed a very beautiful one from the band-stand in the Alfred Park, which is about the highest ground in Allahabad. Just before sunset the sky was more or less covered with high cirro-stratus, and promised one of the very highly-coloured sunsets common in the rainy season, while at the same time a slight storm, heralded by distant thunder, was coming up from the east. After spending a few minutes in the Public Library near the band-stand, I came out, and found the sun had set behind a bank of what Abercromby calls “rocky cumulus,” or some other lumpy form of cloud, and was sending long shafts of alternate light and shadow across the southern half of the sky, while towards the north and overhead the clouds were lighted up with the most gorgeous colours. On turning to the east to see whether the flutings of the cloud-shadows appeared to meet in that quarter, as they usually do, I saw on the approaching shower, which was towards east-south-east, a beautiful double rainbow, both arcs being some 20° long, but stopping short of the horizon by 1½° or 2°, to which height the earth-shadow already extended. Both bows seemed to the eye to be somewhat narrower than usual, and between and beyond them the fluted cloud-shadows appeared, by the illusion of perspective, to converge towards the anti-solar point. The bow must therefore have been produced by the light from a portion only of the sun's disk, shining through a hollow on the top of the western bank of cloud, and doubtless the same portion which illuminated the clouds directly overhead at the time of observation. The rainbow suffered no diminution of brightness where it was apparently crossed by the fluted shadows, the latter being far away in comparison with the bow-producing raindrops, which, of course, were in sunshine.