Types of Iridescent Clouds

Abstract

IN his account of Prof. Størmer's observations on iridescent clouds, Prof. S. Chapman refers to their rapid internal motion.¹ It is perhaps worth while to put on record that this was also a characteristic of the luminous night clouds of the midsummer of 1885, two years after the Krakatoa eruption. I was living in London at the time, at a house in the City, and obtained from the roof a clear view of the northern horizon at midnight, doubtless the clearer for the miles of built-up area between. The belt of brilliantly white cloud (not iridescent) lying just above the horizon was in rapid internal movement; after turning away one's eyes for a few seconds the forms of component wisps were found to be quite changed. Considering the immense distance (probably 400 miles), the movement must have been very rapid indeed. It is obvious that, to be visible from London at midnight, these luminous night clouds were much higher than the iridescent clouds, and their height might well have been 80 km.