Iridescent Clouds

Abstract

SEVERAL brilliant displays of iridescent clouds have appeared here during the past winter. One, on January 19, lasted for more than two hours during a “Chinook” wind. A mass of closely-connected cirro-cumulus clouds formed at a great elevation directly over the eastern base of the Rocky Mountains, and thence extended eastward as far as the eye could reach. The western sky was clear. As the clouds drifted slowly eastward, new clouds formed along their western border. The western limit of the clouds was for several hours nearly stationary, then slowly advanced westward opposite the direction of cloud-motion. Along the western border of the clouds were many projecting tongues of cloud. At one time I counted seven complete spectra at the thinner parts of the clouds—all showing bands of red, green, and violet. There were also about twenty-five spectra showing only one or two of the colours. The larger of these iridescent spots were about 10° in diameter, and they varied in distance from 5° to more than 45° from the sun. Their tints were intensely brilliant. There were also great numbers of minute iridescent spots where the colours were in great confusion—a phenomenon whic his very common here. They sometimes are so numerous as to simulate Bishop's ring.