Unusual Cloud-Glow After Sunset

Abstract

YESTERDAY evening a most extraordinary sunset effect was seen here, which made a deep impression on all who observed it. The sky was nearly clear when the sun set at 4.18, and the air transparent. A few cirrccumulus fleeces became lighted up with a pink and then with a deep red colour immediately after sunset. A very peculiar greenish and white opalescent haze now appeared about the point of the sun's departure, and shone as if with a light of its own, near the horizon. The upper part of this pearly mist soon assumed a pink colour, while the lower part was white, green, and greenish-yellow. About 4.35 the sky from near the horizon towards the zenith had begun to turn to a brilliant but delicate pink, and some pink cirrus-like streaks stretched apparently horizontally towards the south-east. The coloured portion of the sky spread out like a sheaf from the horizon, and apparently consisted of a very high, thin filmy cirrus disposed in transverse bands or ripples, close together, and very delicate in form, outline, and tint. Below the pink, and between it and the point where the sun had set, remained the very curions, opalescent, shining, green and white vapour, hanging, as it were, vertically, and changing very little during many minutes. The borders of the pink sheaf were definite, and finely contrasted with the deep blue sky. As darkness came on, the pink glow seemed to increase in brightness, and at five o'clock cast a fine weird light over the hills. The moon was now bright in the south-east, and began to cast dark shadows. About five the colour slowly receded from the part nearest the zenith towards the horizon, and as it retired left a clearly visible filmy ripple of cirrus of a faint gray tint. At 5.25 the greater part of the colour was gone, and the cloud remained bright only near the horizon. At 5.32, however, it began to grow again, and in a short time (5.40) the whole extent of the film was again glowing bright pink, producing a most striking effect in contrast with the silvery moon, dark sky, and bright stars in the north and east. The pink light then slowly withdrew towards the horizon, remaining bright and deep coloured low down till 5.50. At 5.58 the last pink disappeared. The whole phenomenon from first to last was in the highest degree peculiar and striking. It was remarkable, first, for the interval which elapsed between the time of sunset and the time at which the cloud became bright, next for the light, filmy character of the cloud, thirdly for the bright green glow near the place of sunset, fourthly for the small transverse ripple form of the cloud, fifthly for the permanence of shape and immobility of the cloud, sixthly for the very long endurance of the coloured reflected sunlight after sunset, one hour and forty minutes, and seventhly for the second illumination, which began more than an hour after sunset. It was certainly due to cirrus or a higher kind of cloud, because (1) parts of the illuminated sky stretched in long streaks southwards, and the glow remained long in these streaks, resembling very high cirrus; (2) when the light left the sky the first time, the part which had been illuminated remained visible as silvery gray cloud ripples, before the second after-glow rekindled it, and (3) because the colour became very gradually darker as time went on, and because the recessions of light both times were towards the place of sunset. A similar very high cirrus had also been specially marked long after sunset on November 8, and about the time of sunrise on November 9. The night following this rare display was exceedingly clear and fine. This evening (November 10) the light, high cirrus, all but invisible in full daylight, with its delicate ripples, assumed the pink tint about fifteen minutes after sunset, showing the upper air to be in the same abnormal condition as yesterday, and the phenomenon was feebly repeated. It would be interesting to ascertain the approximate height of cirrus on which sunshine remains one hour and forty minutes after sunset at this time of year.